Nine out of 10 UK mobile phone customers are happy with their service
Some European mobile phone operators have been rounding up call durations to leave customers with higher bills, according to the European Commission.
It has warned operators about charging to the minute, rather than the second.
The warning comes as the UK mobile phone regulator, Ofcom, suggested the domestic market was "flourishing" but needed to be reviewed.
Nine in 10 people were happy with their mobile services, but Ofcom was concerned that complaints were rising.
Only France, Lithuania, Portugal and Spain have legislation that requires operators to charge by the second.
UK MOBILE PHONE FACTS
Network operators: five
Mobile connections: 74 million
UK mobile retail revenues: £15.1bn
Voice calls: 115 minutes per month per subscriber
Number of text messages sent: 59 billion
Source: Ofcom (data from 2007)
The issue might be addressed when the commission publishes plans to cut the cost of calls made abroad, a spokesman said.
"In some cases, operators are charging you for a call of one minute and two seconds what they can charge you for two minutes," he said.
He described it as an "interesting and worrying phenomenon".
In the UK, there are 74 million mobile phone subscriptions - exceeding the population of 60 million.
Some 85% of the adult population have a mobile phone, and they are more likely to have a mobile services from one of the five providers than a fixed-line service.
The industry in the UK is one of the most competitive in Europe, according to Ofcom, generating more than £15bn a year.
Launching a consultation, Ofcom suggested that less regulation might be needed as the access, choice and value for money of mobile services expands.
Nine out of 10 people were happy with their mobile phone service, the regulator said, although complaints were rising.
Mobile phone service agreements were second on the list of the highest number of complaints made to advice service Consumer Direct in 2007, behind second-hand cars. Gripes about mobile phone hardware was fourth.
Ofcom is inviting comments on termination rates - the charges operators make to connect calls to each other's networks - as well as views on broadening access across the UK.
Ofcom's chief executive Ed Richards said now was the time for a strong debate and for some "tough questions" to be asked about the future of the industry.
"Mobile communication is now a central feature of modern life. As our flourishing mobile sector evolves, we want to help maintain strong competition and innovation alongside consumer protection," he said.