British mobile phone users overspent on their monthly phone bills by more than £1 billion last year.
The figures from comparison website uSwitch.com suggest thousands of users have phone tariffs ill-suited to their habits or aren't aware of some of the hidden charges on premium calls.
The research showed nearly half of pay-monthly customers pay more on top of their set tariff every month, with an average overspend of £100 a year - though one in 10 overspend by more than £300.
Premium-rate calls and roaming charges were the most common reasons for overspending, while 12% of responses said calling or texting competition or voting lines were to blame.
More than 10% of pay-monthly users admitted partners or housemates sorted out their mobile bills for them, and almost a fifth have never received itemised bills.
Be switched on
Ernest Doku, telecoms expert at the website, said: "Budget-conscious Brits are signing up to pay monthly mobile deals with the promise of plenty of calls and a set monthly bill.
"However, while it's all well and good having a mobile bundle with stacks of minutes, texts and data, calling premium-rate numbers, voting by text for your favourite music acts and incurring roaming costs when abroad can render them useless, as these are often not included in your tariff.
"If your bills are too high, burying your head in the sand won't help. If you're switched on to your bills - how much you're paying and precisely what you're using - you can make sure you're on the best plan for you, saving money in the process."