If you are switching a mobile service, the process for switching will depend on whether or not you want to keep your mobile number.
Keeping your number when switching is called 'porting' your number, and if you wish to switch and keep your old number, you need to contact your current provider and request a Porting Authorisation Code (PAC). This is a numerical code that you must then give to your new provider.
A PAC must be provided to you immediately over the phone or within two hours by text. Once you have a PAC, it is valid for 30 days. If it expires, you can request a new one. In some circumstances there may be a charge for porting.
Once you have given your new provider the PAC, they will notify your current provider of the port request, and your number should normally be ported and ready to use with your new services within one business day.
Don't forget that if you are still in contract with your old provider, you will have to pay early termination charges.
If you don't wish to keep your number then you will need to contact your new provider to request a new service and contact your current provider to end your existing contract. You may want to coordinate the end date of your old contract and the start date of your new one to prevent an overlap.
Don't forget that if you are still in contract, you need to contact your current provider to give notice that you wish to leave. If you are still within your minimum contract period you may have to pay an early termination charge.