A Consumer's guide to Calling Line Identification services

June 1998


What are Calling Line Identification services?

Most telephone users know that when you dial 1471, you can find out who made the last call to you. This service is called Call Return. It is also possible, if you have suitable equipment, to subscribe to a service where the caller's number is displayed on your phone, or an attachment, before you answer the call. This is called Caller Display. These services were first introduced in November 1994.

Caller Display and Call Return are both services which feature the ability to identify the caller's number and are known as Calling Line Identity (CLI) services. These services can give you more peace of mind when using the telephone and are a welcome development, but there may be situations where callers may be concerned that others may see or find out their number. In most cases, this shouldn't be a problem and callers can get benefits from CLI services, since, if a call isn't answered, it may be possible for the called customer to see or find out the caller's number and return the call. In a few cases, callers will want to maintain the anonymity that the telephone service traditionally used to have, so all customers have rights to maintain their privacy if they feel this is important.



Chapter 1

Chapter 2


Annex A

Chapter 1 of this Guide describes your rights to privacy and the implications of these choices. It covers:

In Chapter 2, more details are described which will be of interest to business customers who may wish to use CLI services to improve the way they use their telephone system and thereby assist their business. It covers:  

Chapter 1

Advice for residential users

Calling Line Identity (CLI) services, such as Caller Display and Call Return, use a feature of modern telephone networks which transmits the number of the caller as each call is set up. Called customers, using these new CLI services, can now have access to this information. With Caller Display, they can see the number on their phone before answering the call, while with Call Return, they can dial 1471 at any time to find out the number of the last caller to their number, whether it was answered or not.

In some situations, callers will want to continue to have the benefit of the anonymity that the telephone system had in former times, so all customers have the right to withhold their number when making calls. In this Chapter, your rights to this privacy are described and the consequences of these choices explained.

Many customers feel more confident about receiving calls now that they can find out who is calling and consequently may feel less confident if their callers use this privacy feature. This booklet also explains what your rights, as a called customer, are in this situation. An important point to appreciate is that if the caller does choose to withhold his or her number, it prevents you from seeing or finding out their number, but it doesn't stop your phone company from having access to this information. CLI has long been used in modern phone systems to allow the phone company to trace malicious or nuisance calls and it also forms an important part of the 999 emergency service, allowing the emergency operator to see the caller's number should the caller be unable to communicate with the operator.

Your rights as a caller

Normally, when you make a call, your number will be available to the called customer, using either the Caller Display or Call Return (1471) services. It is both polite and helpful to others to let them have this information as it can give them more confidence when answering phone calls and they may be able to phone you back later.

However, there may be situations where you don't want those you call to find out your number. The simplest way to do this is to dial 141 before the number you want. So, if you want to dial, say, 0171 634 8700 but want to withhold your number, you would dial 141 0171 634 8700. Always dial the 141 first, in front of any other numbers. However, if you are dialling from an extension on a business switchboard system, where you have to dial (typically) 9 for an outside line, you should dial 9 first, then 141 and then the outside number.

When you dial 141, this is described as blocking your number, or 'blocking your CLI'. As mentioned above, it stops the called customer from knowing your number, but your phone company and their operators can always see it when they need to.

In a very few situations, you may wish to block your number on all calls. Instead of having to dial 141 on each call, you can request your phone company to provide you with a service called Line Blocking of CLI. With this service, all your calls will remain anonymous to those you call. This service is useful in situations where it is vital to maintain your privacy, but there are drawbacks to this service. Some people you call may not wish to answer calls where you have withheld your number or they may feel nervous about doing so. They may have access to a service which won't even connect calls to them if the caller's number is withheld. Therefore you should allow your number to be known wherever possible. In situations where you have elected to have you number permanently blocked, it is nevertheless possible to send your number if you need to. To do this, you need to dial 1470 before the number you wish to call.

When you do allow your number to be accessible to those you call, they can use this information to make a call back to you, so don't be surprised if someone calls you back, even if your original call went unanswered or you didn't give the person your number. People who receive your number in this way may not, by law, use it in a way which affects your right to privacy. When you send your number, it is deemed to be personal data and its use comes within the terms of the Data Protection Act 1984. Annex A provides more information about what your number can and cannot be used for. Because of this, you can allow your number to be accessible, but you can be confident that it is not used for purposes you would not approve of, or be passed on to anyone else.

Some people you call may have their phone service provided by a different phone company from yours. In many cases, your number can still be sent to them, so they can also benefit from accessing your number. All phone companies are bound to ensure that if you withhold your number, this applies on all calls, even ones that pass to other phone companies. So, if you need to withhold your number, you can do so in complete confidence that nobody you call can access it, whatever phone company they may be on.

So, in summary, you have the right to withhold your number, either on a call-by-call basis using the 141 code, or by special arrangement, on a permanent basis. You need to think about the impact of your privacy actions on those you call. It is polite to treat other customers as you would like to be treated, so let others have access to your number whenever possible.

Your rights when called

Most phone companies provide the Call Return service, where you can dial 1471 to find out who last called you. It is usually possible to find out the date and time of the call and, by pressing 3, to make a return call. If the caller withheld his or her number, you will be informed of this. 1471 calls are free of charge, but if you use the return call feature, your call will be charged in the normal way.

Sometimes calls are not able to deliver a number for the caller, for example on some calls from older mobile phones, on international calls, or calls from other phone companies that have not implemented any CLI services. On such calls you will be informed that no number is available.

[In future, you will be able to dial a special code to erase the record of who called, so that others who have access to your phone and who might later dial 1471 cannot find out who called you.]

Many phone companies also provide the Caller Display service, for which a rental charge is made. To use this service, you will also need to buy a suitable telephone, or display attachment, which will allow you to see the number of calls before you answer them. Such telephones usually have a memory feature to keep a record of a series of calls made to you. When buying a Caller Display telephone or display attachment, you may need to check that it is compatible with your phone line, as some phone companies use a slightly different arrangement to deliver CLI numbers. However, many display phones can work on all UK networks.

The advantage of the Caller Display service, over using 1471, is that you can see who is calling you before you answer, so you can chose not to answer the call, if you wish, or perhaps leave the call to be answered by an answering machine, if you have one. When you use the Caller Display service, your phone will display not only the number, but also the date and time of the call. It will also usually display the text WITHHELD or UNAVAILABLE when a number cannot be delivered. The phone may be able to associate the number received with a directory of names of your frequent callers which you have stored in the phone's memory.

Occasionally, other words may appear, such as PAYPHONE, INTERNATIONAL or OPERATOR. While this can be helpful in explaining why the caller's number is not available, you should be aware that this text is not always consistently displayed on calls from payphones, from abroad or from the operator. In future, callers' numbers may become available even on international calls.

When you receive callers' numbers, you may use them to decide how to handle the call or to make return calls. But as explained above, these numbers are personal data and you must not use the information for other purposes unless the caller has given his or her permission. Annex A gives further information on this.

In some circumstances, you may find that you receive more calls where the caller's number is withheld than you are comfortable with. If these are sales calls, you may wish to contact your phone company about registering under the Telephone Preference Scheme, which should reduce such unwanted sales calls. However, for other situations, your phone company may be able to provide a service which prevents 'withheld' calls from getting through to you. With this service, callers who withhold their number hear an announcement saying that you do not wish to receive calls where the number is withheld. Calls, which for other reasons, cannot provide a number, that is, where the number is 'unavailable' are not blocked by this service. In the near future, more phone companies will be offering this service. It is known as Anonymous Call Rejection, or sometimes 'Block Blocking' because it allows you to block those calls where the caller has blocked their number. You need to think carefully before opting to use this service, as many customers withhold their number for quite innocent reasons, especially in cases where no return calls can be made. One example is a hospital payphone on a bedside trolley, but there are many others.

Where numbers are available for display or access via 1471, you can have confidence that they represent the number of the person calling you. In some cases, where calls come from businesses with private switchboards, the number displayed might be the number of the switchboard operator, rather than the extension calling you. In other cases, the actual extension number may be displayed. There may be instances where the number is different from the actual calling line: this is known as a Presentation Number. These are used when the actual calling line is not appropriate for receiving return calls, for example when a call is made on an "outgoing only" line or when an organisation uses different phone companies for outgoing and incoming calls.

However, Presentation Numbers are only allowed in controlled circumstances and such numbers must belong to the same person or organisation as the actual calling line, so you can be confident that the identity of the caller or his company is always correct.


Chapter 2

Advice for business users and telecoms professionals

This chapter is designed to give you more information about CLI services, to allow businesses and professional telecoms people to make the most of these services. It is advisable to read Chapter 1 first, as this contains important advice for all telephone users.

As a business customer, you may be using more complex telephone equipment than the residential customer, such as ISDN terminals, mobile phones, Private Automatic Branch Exchanges (PABXs, referred to in Chapter 1 as 'business switchboards') or Call Centres.

Choice of telecommunication networks

As a business user, you might well be using, or intending to use, one of the many competing public telecom operators (PTOs). If CLI is important to you, then you should be aware that CLI (when used for display purposes) is an optional service and PTOs are not obliged to provide it. You should therefore ask any potential PTO provider whether, and what, CLI services are supported.

ISDN and mobile networks

Calls between compatible ISDN equipment have been able to transfer CLI numbers even before similar services were launched for the residential consumer in 1994. As such they should conform to ISDN standards. In some cases, ISDN systems may not be able to make the distinction between CLI being unavailable or being withheld and you should bear this in mind when receiving calls over such systems. The new Euro-standard ISDN does support the distinction. However, in most cases, originating ISDN lines can use both 141 and the standard ISDN blocking code (1471) to withhold the number. ISDN standards refer to the display of CLI as CLI Presentation (CLIP) and the blocking service as CLI Restriction (CLIR).

CLI on digital mobile networks such as GSM and PCN works in a similar way to ISDN.

Presentation Numbers

In order to explain some of the CLI options in greater detail, it is useful to appreciate how CLI is generated and transmitted across public networks. There are two forms of CLI. Firstly, there is the Network Number (NN) which is set by the public telecom operator (PTO) and usually corresponds to the regular directory number of your line. Secondly, there is the Presentation Number (PN) which is again set by the PTO to be a number on which you wish to receive return calls, if for some reason it is not possible or desirable to have return calls to the Network Number.

If you wish to use a Presentation Number, you should contact your PTO about this. You may only use a PN which corresponds to a line which is rented by the same person or company, so you may not, for example use a third party Call Centre number. The number may not be a premium rate information number, but all other numbers are allowed as long as they are diallable. There are several reasons why you might want a PN. For example, your lines may be 'outgoing only' and you wish return calls to go elsewhere. Alternatively, you wish all return calls from a group of lines to go to one enquiry number. If the PN corresponds to a line which belongs to another PTO, the PTOs will liaise to ensure that the PN is valid and in your name.

Both NN and PN are carried across the network as calls are set up, including calls which pass to other PTOs. At the distant exchange, customers using Caller Display or Call Return are sent the PN, if one is available, otherwise the NN is sent. Note, however, that there are a few PTOs that cannot handle PNs . If you use a PN, you should advise your PTO whether you would prefer, in such circumstances, for your NN to be sent, or the CLI shown as 'unavailable'. This preference would only apply where the distant PTO cannot handle PNs, in all other cases, the normal PN would be made available.

CLI on PABX systems

If you have a PABX with Direct Dialling In (DDI), you will need to consider carefully how you use CLI. Where you connect to the PTO with a digital line (usually one capable of carrying ISDN), it will usually be possible for you to transmit the number of the calling extension to the PTO. In this case, the PTO will transmit a PN comprised of the known number stem of your PABX with the extension number suffixed to it. The NN will usually continue to be set to the switchboard number. Thus customers you call should be able to see or access the number of the actual extension which made the call and return calls to it. This avoids return calls to the switchboard operator who will in general be unaware of who might have called a particular customer. It is also useful to prevent an increase in calls to the switchboard, as you might have to employ more operators.

With this system of CLI, the PTO will verify that the extension number you send is a valid one within the agreed range. In this way, customers can remain assured that the CLI is authentic.

If you have a network of PABXs, you may be using a feature usually known as 'far end breakout'. This is a way of routing your outgoing calls to the public network, where the call routes first to one of your distant PABXs before connecting with the PTO, in order to take advantage of local charge rates from that point. In this instance, the second PABX will not be able to send forward the extension number of the calling extension, even if it is known. This is because the extension number would not correspond to the number stem stored by the PTO as the Partial CLI. Because of this problem, many customers with such private networks have chosen to withhold their CLI permanently, rather than allowing the default switchboard number to be seen by the called customer.

This is progressively becoming a major problem and with the widening availability of Anonymous Call Rejection (ACR) services, it may be necessary to allow large private networks to transmit complete PNs, rather than the present system of sending only the extension number. This is currently being studied by the CLI Interest Group and if you are such a large business user with this difficulty, you may wish to contact Oftel for further information or to join the CLI Interest Group. Were such complete PNs to be allowed onto the public network, it will be important for the private networks to be bound by the same rules of CLI privacy, authenticity and integrity as public networks.

Indirect Access to other PTOs

You may subscribe to the services of an alternative PTO, usually providing long distance or international calls. In some instances, you dial a short code or an 0800 number, or press a special phone button and are then invited to identify yourself to the second PTO using your touch-tone keypad, before entering the wanted number. This is called two-stage call set-up. When you use two-stage call set-up, the second PTO will not necessarily be aware of your privacy preference, that is, whether you have dialled 141 or have your CLI permanently blocked. Indeed, on the older Mercury blue button phones, it is not possible to set up a call via the Mercury (now Cable & Wireless) network if you dial anything (such as 141) before you press the blue button.

In other instances, it is possible to dial the short access code (eg 1XXX) immediately followed by the wanted number. This is called one-stage call set-up. In these cases, the second PTO identifies who is calling by using the Network Number CLI and this allows the PTO to bill you correctly for the call. This has a number of implications you should be aware of.

Firstly, you may withhold your number using 141, as usual, if you don't want your number to be available to the called customer. The second PTO still has access to the NN for billing purposes.

Secondly, you may not access your PTO from a telephone line different from your normal one, as the CLI will not correspond to the one recognised by the second PTO. If you need to make calls from varying lines, ask your PTO about Calling Card services.

Lastly, if you use Call Diversion, it is unfortunately not possible to divert calls using your second PTO's indirect access service. This is because, on a call which has been diverted, the CLI sent forward on the diverted leg of the call is the CLI of the original caller whose call has been diverted, and not yours. Thus the indirect access PTO will not recognise the CLI as a valid number for your account. The reason why CLI works this way, is to prevent callers being able to send a false CLI number by diverting via a third party. It is important that the CLI accessible to customers always represents a diallable number to which return calls can be made to the caller.

Smart Boxes

You may be making use of indirect access PTO services by means of a so-called Smart Box between your system and the public network. This is programmed to route calls to the alternative PTO by inserting the appropriate indirect access code. You should be aware that many such Smart Boxes route all calls beginning with the digit 1 to the regular directly connected network (usually BT). Thus if you withhold CLI using 141, the call will default to BT. If you need to withhold CLI on a majority of your calls, it would be best to have your line permanently blocked for CLI and use 1470 when you wish to release CLI. (Of course 1470 calls would also default to BT)

Call Centres

Many Call Centres operate within the UK, and often use the received CLI, via Computer-Telephony Integration (CTI) systems to alert the call centre operator to who is calling and, where the customer is already a known contact, to bring up stored information from a computer to assist with the handling of the call. This is an acceptable use of CLI, but in other circumstances, captured CLI numbers may not be used for more general marketing purposes, such as compiling target lists for an outbound calling programme. Annex A gives further information from the Data Protection Registrar about acceptable uses of captured CLI.

Service Providers

As mentioned above, the PTOs transfer CLI (both NN and PN) between themselves, irrespective of whether you have requested it to be withheld from those you call. This is necessary for a range of reasons, including billing, call tracing and the handling of operator and 999 calls. To ensure that your privacy preferences are maintained, all PTOs are bound by a Code of Practice, backed up by their interconnect contracts, to ensure the authenticity and integrity of CLI and its privacy indicators, be it 'withheld' or 'unavailable'.

Independent (non-PTO) Service Providers connecting to PTO networks cannot access any CLI if the caller has withheld it. In some cases, it may be important for Service Providers to have access to such CLI for management and billing purposes or if it needs to be passed on to another PTO for final call delivery. In the future, it may be possible, in certain circumstances, for Service Providers to be able to see the CLI as PTOs do now, subject to binding contracts concerning the maintenance of privacy and integrity of the CLI information. Service providers who are interested in this feature should contact Oftel in the first instance.

The CLI Interest Group

If you have an interest in debating and assisting future policy for CLI services, you may wish to join the CLI Interest Group. This is open to all industry members, consumers and their respective representative organisations. It is facilitated and chaired by Oftel. For further information, contact Frank Phillips on 0171 634 8871.



anonymous call rejection (ACR): a service which prevents calls from being connected or put through where a caller has withheld CLI

block blocking: same as ACR, the blocking of calls where the caller has blocked or withheld CLI

call return: a service which enables a call to be returned to the line from which the last incoming call was made

caller display: a subscription service which displays the number from which a call has been made - it requires a caller display unit with a screen, either on the telephone or as a separate attachment

computer telephony integration (CTI): the integration of screen-based computer functions with telephony

direct dialling in (DDI); a switchboard's capability to route an incoming call to the extension dialled, without the intervention of an operator

ISDN: Integrated Services Digital Network - the most widely available digital network, providing a wider range of services than an analogue network can support

GSM: Global System for Mobile Communications - a digital mobile communications system operating around the 900 MHz frequency band

line blocking: a service which enables a caller to withhold CLI on all outgoing calls

network number (NN): a number allocated by a telephone company which identifies the line from which a call has been made, usually the same as the directory number

PABX: Private Automatic Branch Exchange - often described as a switchboard and switches calls between exchange lines and extensions

PCN: Personal Communications Networks - a digital mobile communications system operating around the 1800 MHz frequency band, also known as DCS and GSM 1800

presentation number (PN): a number chosen by a caller to which return calls may be made

telephone preference scheme: a voluntary scheme by which subscribers register with their telephone company their wish not to receive any telemarketing calls




Using the law to protect your information

Calling Line Identification (CLI)

Caller Display - Use of Captured Data

1. By Caller Display we mean the facility which provides for the display and possible capture of the caller's number by the called party.

2. In the implementation of Caller Display concerns about protecting the privacy of the caller have been recognised and the caller is able to suppress the display of his number on a call-by-call basis by dialling the prefix 141. Suppression is possible for all calls made from a particular number.

3. Caller Display allows not only the display but the capture of the caller's telephone number by the called party. If that number is subsequently held as personal data by the called party, the question arises: has it been obtained fairly?

4. It is a requirement of the First Data Protection Principle that information to be contained in personal data is obtained fairly. The Registrar's view is that this means that the source of the information should know who is to use the data and for what purpose or purposes the data are to be used or disclosed. If these matters are not obvious because of the nature of the transaction, they must be explained to the source before the information is given. This view has been supported by the Data Protection Tribunal in its decision regarding Innovations (Mail Order) Limited.

5. There are a number of reasons why the calling number might be retained by the called party. These range from use by a private subscriber simply to return the call, to commercial use such as enhancing an existing customer database or establishing a new database.

6. The Registrar would accept that use of the captured number in direct connection with the original call by the called party is a use which the caller would reasonably expect. Such use could include the example above of returning a call if the called party had been unable to take the original call immediately. (in any event, in this case it is arguable that the information retained is not personal data). Another such use, in a commercial context, would be to reference an existing customer database if the number were already held on it.

7. However, the Registrar considers that obtaining the number and holding it as personal data if it were not already held as such is something which may not be obvious to the caller. In those circumstances, it is likely that the fair obtaining requirement of the First Principle would have been contravened by the called party if no explanation had been given to the caller.

8. A further concern is that individuals may not often ring companies from home. Therefore, it is dangerous to assume that you can call a specific individual back on the number from which they have made a call. Where a company wishes to use the number displayed to get back to a caller a sensible, practical safeguard is to ask a caller if you can call them back on the number displayed.

"Do you mind if we call you back on 01 793 056708?" (or whatever the number displayed is).

9. It is easy to envisage circumstances in which the calling number could be retained for purposes which are unconnected with the original call. An example would be to enhance a marketing database which is intended to be rented to other companies for their marketing purposes. The Registrar considers that to retain the number for such purposes without the express consent of the caller, would be likely to contravene the First Data Protection Principle.

10. In summary, the essence of the matter is that the callers number may only be captured and held as personal data by the called party for purposes directly connected with the original call, unless the caller gives express consent to its subsequent use for other purposes.

Data Protection Registrar

14 August 1997

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