Lots of companies compete to offer you telephone and broadband services.
Companies use different sales and marketing tools to attract customers. While most of these are done responsibly, some companies use dishonest activities such as mis-selling.
One form of mis-selling is known as ‘slamming’, where customers are switched (or attempted to be switched) from one company to another without their knowledge or consent.
You might only become aware of this after receiving a bill from a different company.