EE, Three and Vodafone are charging customers extra for mobile phones that have already been paid off, according to a report by Citizens Advice.
Customers who don't start a new contract once their phones have been paid for are shelling out an extra £22 a month on average, the report says. If you've got a premium phone such as a Samsung Galaxy or Apple device, you could be paying up to £38 more than you need to every month.
Government minister for digital Matt Hancock responded to the report, saying mobile phone companies need to notify customers when their phones have been paid for.
However, the mobile firms dispute these findings.
"Whenever a new customer signs with us, we make the end-date of the contract term very clear," a Three spokesperson said. "We encourage all Three customers to contact us if they would like to change their plan at the end of their fixed term deal."
EE said they send customers regular updates about their options before and after they reach the end of their contract and that, "the vast majority of our customers upgrade to a new phone or move to a SIM-only plan near the end of their contract."
A spokesperson from Vodafone said: "We strive to give our customers the price plan that best suits them. Wherever possible, we contact our customers nearing the end of their contract to offer them a range of options. These include being able to upgrade their handset, receiving an extra allowance to enhance their existing plan or, if they choose, switching to a SIM only plan."
However, Nina Bibby of O2, one of the UK's most popular networks, has spoken critically of the billing practices.
"Forcing customers to continue to pay for a phone they already own not only hits their pockets but undermines trust and the reputation of the industry," said Bibby in a statement. "We'd like to see the other operators review their position and follow our lead."
Update, Oct. 20 at 11:08 p.m.: Adds comment from Vodafone.
Update, Oct. 20 at 12:06 p.m.: Adds comment from O2.