Average cost is an eye-watering £213 per customer.
With the summer drawing to a close, it seems that a large number of UK mobile customers have been hit with unexpectedly large phone bills following trips abroad.
A survey of over 3,000 British mobile users found that 54% of customers from three of the UK’s biggest mobile networks – EE, Vodafone and O2 – have suffered "bill shock" this year.
The average cost was a pocket-draining £87.70 a month, multiplied by the average customer having experienced this 2.43 times, leading to a total of £213 for each customer.
The costs mainly came from high and unexpected roaming fees and charges for dialling 0800 numbers, which are not covered by the operators, and can include calls to insurance providers or healthcare companies.
The research also revealed that nearly a quarter (23%) of customers from O2, Vodafone and EE combined have not switched mobile operators due to the potential costs incurred in buying themselves out of their current contract.
The survey came from mobile network Three, whose "Feel At Home" initiative, launched last August, does away with roaming charges in select countries. Users visiting 16 territories across the world, including the likes of France, Australia, and the United States.
Should users exceed their allowance, calls, texts and data are charged at lower roaming rates.
"With over half (53%) of Brits stating that getting good value from their network is a key factor to them moving, it’s surprising that so many people are still holding on to the network that has left them angry and frustrated instead of switching," said Tom Malleschitz, Three director of marketing.
"It’s important that people should be treated fairly by their mobile network provider, so we want to encourage people to take action and move to an operator that offers them what they want."
The findings come weeks after the success of a long-running EU campaign to end roaming charges across the continent, with the price caps for data downloads, phone calls and text messages significantly cut since July 1.
Led by EC Digital Agenda head Neelie Kroes, the European Commission also passed legislation earlier this year which will abolish roaming charges across Europe by December 2015.