Roaming services have now been fixed for all customers on the Three mobile network in France, Portugal and Luxembourg, the operator has said.
Three apologised for the issue, which had lasted since yesterday.
On Twitter, several customers complained about problems they had experienced.
"The issue with our roaming partners which was affecting roaming service in France, Portugal and Luxembourg has now been fixed," said Three in a statement.
"We apologise for any inconvenience."
One customer wrote online: "I'm travelling alone and can't make any calls or send any texts."
Another said: "I'm driving to Paris tomorrow, and I've got to follow road signs because I have no connection for my Google Maps."
BBC journalist Dougal Shaw - on holiday in France - also said on Twitter that he had been affected.
"I got lost in a market," he wrote.