Millions of EE mobile customers are facing steep price hikes from next month for calls and messages made outside their pay monthly price plans.
The increases will see the cost of standard UK calls jump from 40p to 50p a minute, but the biggest hike is for international calls to destinations including the US and Canada, Australia and New Zealand, which will cost £1.60 a minute instead of the current £1.
Outgoing SMS messages from the UK to international destinations will increase from 25p per text to 35p.
The changes come into effect on September 28.
Pay-as-you-go charges are unaffected by the change.
Ofcom regulations allow customers to leave their contract without penalty if the monthly tariff increases without a warning at the time of signing up, but the protection does not extend to out-of-allowance increases.
EE's move follows Vodafone increasing its out-of-allowance charges for pay monthly customers last month.
MoneySavingExpert.com (MSE) managing editor Guy Anker said: "Clearly these are some very steep hikes that are going to mean big additional bills for some people.
"This is a wake-up call for every mobile customer, no matter who their provider is. All providers tend to put up their prices at least once a year."
But he added: "Calling abroad these days doesn't have to be the big expense it once was.
"Skype, FaceTime and WhatsApp can all let you do the same thing for free."
MSE also offered tips to consumers on how to cut costs, such as increasing their UK call allowance, calling international numbers via the internet, seeking out alternatives to 084/087/09 and 118 numbers and sending pictures for free online.