An estimated four million people have been charged £490 million for handsets they already own, a charity has said.
Citizens Advice say EE, Three and Vodafone routinely continue to charge customers for handsets after they had paid them off in full. So how can you make sure you are paying the best price for your handset and contract?
Renegotiate your deal
If you're keen to stay with your current network once your phone contract has ended, you can renegotiate your deal and switch to pay-as-you-go.
Switch network providers
At the end of your contract you can switch network providers and get a SIM-only deal.
Similar to a phone contract you would be paying a monthly cost for minutes, texts and data. However, the price should be substantially lower as it won't include a new phone.
If you wish to keep your number when switching networks, you'll need a PAC code from your old provider.
Avoid unexpected phone charges
Calls to 09, 0871, 0872, 0873 and 118 numbers will add up on your phone bill.
Here's an a rough guide of what these calls cost.
If you find an unexpected phone charge on your bill you can dispute it by calling the phone-paid services authority.
Here are some best practices to avoid racking up the unexpected costs in the first place.
If offered insurance for your phone check to see if your home insurance or bank account policy already covers it.
Once you're due an upgrade Citzens Advice suggest asking network providers to check your phone bills from the last six months.
While unlimited data and text may seem like an appealing option if you're not making full use of it, choosing the right allowance can help save money.