Vodafone is raising its prices by 2.4% from 1st November.
Pay monthly line rental for contract customers will go up and the cost of calling premium rate numbers will also be affected.
The changes are likely to cost most Vodafone mobile phone customers an extra 59p a month or £7 a year.
The company blamed rising costs for the increase, and argued it was at least keeping the price hike below inflation:
"Like most businesses in the UK our costs are rising. So we need to review our prices now and again. We've made every effort to minimise the impact and have kept this increase below inflation," it said.
But consumer group Which? has slammed the decision and is now stepping up its campaign to keep fixed contracts at a fixed price.
Fixed should mean fixed
So far this year, four out of five of the main phone operators have exploited a clause that allows them to increase prices on contracts that appear to be fixed.
Orange raised its prices by 4.34% in January, T-Mobile followed suit with hikes of 3.7% in May, Three was next with an increase of 3.6% in the same month and now Vodafone's increases are on the the cards for November.
Usually, most contracts state that as long as the increase isn't higher than inflation, companies can jack up prices and no-one can do anything about it. For more read: Mobile networks using small print to raise prices on fixed tariffs.
Which? reckons that these sneaky tactics could earn providers an extra £90 million this year.
In particular, the consumer champion has estimated that Vodafone has made up to £10.5 million in revenue from price increases made to its 'fixed' contracts since last year.
In October 2011, Vodafone caused outrage when it increased prices by rounding bills to the nearest 50p.
What you can do
Which? has lodged a complaint with the OFT and so far has gained over 28,000 supporters.
If you want to pledge your support to make the phone regulator take notice join the Which? Fixed Means Fixed campaign.