If your mobile is on a prepaid tariff, or you regularly use more than the number of minutes included in your contract, you will know just how expensive mobile call charges are.If you are a pay-as-you-go Vodafone or Orange customer, they are about to go up.From Friday Orange will up its minimum call charge for pay-as-you-go customers from 20p to 25p per minute - a 25% increase.Meanwhile, pay-as-you-go customers on Vodafone will see an increase in the cost of a text from 10p to 12p.Call charges to both mobiles and landlines will go up from 21p to 25p.Mark Newman from Informa told the BBC only a handful of people will be affected by these changes.