EE customers have been sent billing statements amounting to tens of thousands of pounds after falling victim to a data roaming glitch.
In some cases, account holders have discovered eye-watering charges on their online accounts, while others said EE had blocked their phones due to the system error.
The issue, which started last week, has affected users travelling in America, Canada, Mexico, Australia and New Zealand. It meant some £10.44 data passes for use abroad had failed to activate.
Darren Steinnman, 33, from Pontefract, West Yorkshire, was sent a bill for more than £1,000. His account was later suspended.
Mr Steinnman, a Royal Air Force employee now based in Las Vegas, said: "I bought a roam further pass before I came to the USA.
"However, I received a message from EE saying my account has been suspended due to high usage.
"I couldn’t phone EE or anything as my account was blocked. In total the excess data charges were well over £1,000."
Another said they had been charged £300 for 40mb of data, despite buying a roaming add-on for the USA.
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A separate person told The Telegraph they had been incorrectly charged £25,000 after a month in the US.
Bill payers had received text messages telling them to clear their debts or network access would be cut off.
Blocked customers still cannot not use their phones, despite contacting EE to rectify the issue. Some have had to wait more than a week.
Elizabeth Williams, 38, from London told the publisher she was on a four-day business trip to Los Angeles when she received a text message saying she had built up £500 in charges.
“It said I needed to pay or my phone would be shut down. It was a massive shock at six o’clock in the morning while I was relying on my phone for calls, meetings, emails and directions,” she said.
“I rang them up straight away. They told me the issue was fixed but the charges are still showing up,” she added.
EE, which claims to be Britain’s best, network, has apologised to those affected.
It said it is in the process of contacting customers to amend their billing documents.
A spokesman said the “problem” linked to roaming passes abroad has now been fixed.
“We’re very sorry for the problem affecting a small number of customers using their Roam Abroad pass last week. The issue was quickly fixed within 24 hours so customers could continue to use their Roam Abroad pass as usual and the extra charges are being removed from their accounts."
Consumer expert Martyn James at Resolver encouraged customers to check their accounts on the app or online.
"If you've been abroad in the past month and purchased a roaming pass, don't assume you've been billed correctly.
"Check your bill urgently to make sure you haven't been overcharged and if you're not sure, get in touch with the business and get them to explain anything that looks wrong."
He added that the charges are a bleak reminder of how bills will change when roaming fees return.
"The dark days of data roaming charges may be a distant memory for most, but these spectacular billing errors reveal just how high the cost could be for using your phone abroad."
Martyn said: "While some of bills sent out by EE are patiently wrong, data charges while in a different country can still be eye-wateringly high - and you can easily be hit with a massive bill just be making a few simple errors with your phone's settings.
"Most mobile phone providers have stated that roaming charges will apply again in Europe this year, so beware! The cost of using your phone for scrolling through social media on the sun lounger could be more than the cost of your flights."
