The consumer watchdog Which has issued a complaint to Ofcom after finding that mobile networks are using fine print to raise fixed tariff prices.
Which reckons the likes of Three, Orange and Vodafone have pocketed up to £90m by hiking up the fees during existing contract agreements.
According to the organisation, which has launched a new campaign called Fixed Means Fixed 70 per cent of customers are unaware networks can uses these "hidden clauses" to increase fees.
"These hidden price rises mean millions of people are forced to pay more than they expected at a time when household budgets are already squeezed," said Which? executive director Richard Lloyd.
"They are then trapped in a contract, unable to switch to a cheaper provider without paying a hefty penalty."
Three singled out
Which? points out that Three is currently in the process up hiking up all contracts agreed before March 8th 2012 by 3.6 per cent. Three says it is to account for inflation.
The company said in a statement: "Our terms and conditions allow us to raise prices in line with inflation so that we can cover our business costs."
Ofcom says it is investigating the issue and will take the advice from Which? under consideration.
A spokesman said: "We understand why consumers in fixed term contracts are sometimes disappointed to find that the particular contract they have signed up to allows price rises.
"While current rules allow for contracts to include price increases in certain circumstances, after receiving consumer complaints on this issue, Ofcom launched a review in January 2012."