Shock bills, phantom charges and billing errors are driving people into debt, and phone companies are "hanging customers out to dry", according to Citizens Advice.
Its consumer service dealt with calls from 28,000 people experiencing problems related to mobile phones and contracts across England, Wales and Scotland in 2013. And a further 102,000 people sought help online.
Some consumers had been hit by sky-high phone bills after making calls abroad, using the internet or because they had been victims of scams and had their phones stolen.
One lady who called the charity received a £2,000 bill after she travelled to the US. She had used her smartphone and a hotel's Wi-Fi to keep in touch with her family. She was forced to pay the bill as her mobile phone operator insisted she had not turned off data roaming or bought an international package before travelling.
While in the EU there is a limit on how much phone users can be charged for data, with fees capped at 40p (€0.45) per megabyte. But Citizens Advice wants the mobile networks to extend the cap to cover countries further afield.
It also wants them to put a stop to people being tipped into the red by unexpectedly high bills.
Driven into debt
Gillian Guy, chief executive of Citizens Advice, said: "Mobile companies are hanging their customers out to dry with shock phone bills. Some people are facing bills of hundreds or thousands of pounds. Others are being driven into debt as they struggle to cope with these unexpectedly large bills.
"Phone providers could help people by sending them text messages with reminders about the costs and any limits they have. There is also an opportunity for firms to be innovative by creating tools for people to keep day-to-day track of their charges, calls and data use."
Consumers can also take steps to steer clear of running up a large bill abroad including checking costs with their network before they travel or getting a local sim card if you visit a place regularly."
If you've need advice about a mobile phone issue, contact the Citizens Advice consumer service on 08454 04 05 06.