Variable pricing in long-term mobile phone contracts
16 July 2012 Last updated at 11:27 BST
When people want a mobile phone contract they will either sign up for Pay-as-you-go, or take out a long-term contract with a network provider.
But many customers don't realise that monthly payments can go up during the course of the contract.
Customers of the 3 Network on Monday are witnessing a 3.6% rise in their tariff, following similar rises from other companies.
The consumer group Which? has said that up to 10m customers could have been affected by price changes in phone contracts.
Clare Francis from MoneySupermarket discussed the pricing scheme on BBC Breakfast News.