Vodafone says it doesn't believe the changes will affect customers much
The National Consumer Council (NCC) has criticised Vodafone for increasing mobile phonecall costs without telling its customers.
Vodafone plans to raise minimum call charges by 25%.
But a letter inserted into July's bills stated the new price list but failed to mention they were going up.
Vodafone has said it has improved its call packages for contract customers and it expects "99% of users will see less than a 10% rise in their bills".
It says it needs to raise prices to cover the cost of connecting to non-geographic numbers such as 0845 and 0800, and because the European Commission has made firms cut the cost of calls made from overseas.
Vodafone is not changing its headline tariff rates and the increased charges, effective from 1 September, apply to both contract and pay-as-you-go customers.
Minimum call charges are rising by 25% and calls to 08 numbers, including 0800 and 0845, will be rising by more than 30%.
Vodafone said it let customers know that changes were afoot through texts and their bills.
The company says its new improved price plans, with extra texts and minutes for contract customers, will help absorb some of the increase.
A Vodafone spokeswoman said: "This is the first time in over two years we have done this and 02 and T-Mobile have already done it earlier this year.
"If people are significantly impacted, we will work with them to find the right price plan."
The NCC has called on mobile phone providers to make sure they are absolutely clear on prices.
Last year, the Commission set limits on roaming charges for mobile phonecalls across the European Union.
At the time, mobile phone companies warned that prices of other calls may go up.
In June, the Commission unveiled plans to lower the cost of mobile phone calls by reducing the fees operators charge each other for using their networks.
The Commission said it was these charges that were, in part, to blame for mobile calls being more expensive than fixed-line calls in the EU.