Vodafone has apologised after an error meant customers using the mobile network abroad were hit with roaming bills of up to £10,000.
Customers took to Twitter, saying they were unable to use their phones and could not reach the operator.
Many received alerts from Vodafone that their data had run out, despite the fact that many still had data remaining in their monthly allowances.
Vodafone said the issue was caused by a technical error.
Since 2017, under EU regulations UK consumers are able to use the minutes, texts and data included on their mobile phone tariffs when travelling in the EU at no extra charge.
Vodafone customers posted on Twitter that their bills had risen by between hundreds and thousands of pounds within just 12 hours.
'This is a joke'
Andy Pearch told the BBC that on Sunday, just 24 hours into his holiday in Malta, he had a shock when he received a text message from Vodafone informing him that he had spent £4,902.75 in "additional charges".
To prevent him from incurring an even larger bill, Vodafone said it was preventing him from making any calls, sending text messages or using mobile data on his phone.
But when Mr Pearch logged onto his Vodafone account using the hotel Wi-Fi, it showed that he still had 14.2GB of data remaining out of his 20GB monthly data allowance.
"Stress is not the word for it," Mr Pearch said. "My service was cut off from 22:00 last night till about 08:00 this morning."
Regional sales manager Kevin Navette is currently abroad, but he has been blocked by Vodafone from using his work phone.
Mr Navette told the BBC that he has been charged £3,000 and his service stopped working on Sunday. However, despite contacting Vodafone on Twitter on Sunday and Monday morning, his service is still not working.
"This is my professional phone so it's big trouble for me," he said.
Vodafone said: "We are very sorry that some customers could not use their phones yesterday, when roaming abroad. This was due to a technical error, which we have now fixed.
"Some customers are receiving billing messages in error; we are working through these as an urgent priority and are removing any errors from customer accounts. Customers will not be charged and do not need to worry about contacting us as we are proactively checking accounts."
In August, a similar issue affected Three Ireland. The mobile operator had to apologise after a system upgrade error added bogus roaming fees to customers' bills and suspended their services.