Millions of mobile phone users regularly pay extra charges when they do not need to, according to a consumer watchdog.
Which? Mobile found an estimated 10 million users pay for extra calls, texts and data usage on top of their monthly line rental.
The consumer group said the findings were "not surprising" considering an estimated six million people did not know, or had only a vague idea, of their monthly limit for call minutes, while five million were unsure of their text and data allowances.
However, seven in 10 people claimed to check their mobile bill at least once a month.
Which? said consumers could save money by switching to a tariff that better suited their phone usage, but 18 million people (39%) had never switched mobile phone provider.
Tom McLennan, head of Which? Mobile, said: "You could be spending hundreds of pounds more than you need to on your mobile every year if you're not on the right tariff.
"If you regularly spend more than your line rental, check your bill to see where you're incurring the extra charges as you may be able to save money by moving to a tariff with more minutes, texts or data.
"If you never go over your agreed line rental, it's because you're not using up your allowance so you may be able to save money by finding a cheaper tariff that reflects your usage."
* Which? surveyed 1,271 adults about their mobile phone contracts in November.