Three Ireland has apologised after adding bogus roaming fees to customers’ bills.
In some cases, subscribers had been told they had been charged more than €160 ($179; £147) for large amounts of data use that exceeded their allowance.
Many were alerted to the charge by a series of text messages from the firm, and then found that their service had been suspended as a consequence.
The company has blamed “a system upgrade error”.
It added on social media that it was currently trying to identify which customers had been affected and would refund them “in due course”.
Some users have complained that they have been told they will have to wait several days for the money to be reimbursed.
Others said they had to wait hours in line to have their calls to Three’s support service to be answered, because there were hundreds of others waiting in the queue in front of them.
Some users reported continuing to have received the text warnings on Thursday morning.
But a spokeswoman for Three said the problem had now been dealt with.
“Due to a system upgrade error, a number of customers may have experienced issues with roaming charges yesterday,” she told the BBC.
“All service has now been fully restored.”