EE has been censured by Ofcom for overcharging people who rang its customer service number while roaming in the EU.
The watchdog believes there are "reasonable grounds" to believe people who rang the customer service number 150 were charged with US roaming rates between July 2014 and July 2016, even when they were within the European Economic Area.
However, this is not a final ruling and EE will now have the opportunity to respond to Ofcom before any final decision is made.
Nevertheless, if the ruling goes against EE, it could potentially be forced to pay a sizeable fine.
The company has insisted that the issue with call charging arose as a result of a "technical billing error" and said measures to prevent it happening again were put in place as soon as it became aware of the problem.
"We have already identified and credited the vast majority of customers who were affected, and we apologise for any inconvenience this has caused," EE said.
This comes shortly after Ofcom revealed that EE was the subject of 21 complaints per 100,000 subscribers during the second quarter of 2016, just behind Plusnet and BT.
Lindsey Fussell, Director of the Consumer Group at Ofcom, said: "We expect providers to make customer service and complaints handling top priorities. While complaints have gone down, providers must work harder to better serve their customers.
"Consumers have a right to expect good service and will rightfully complain when that standard isn't met. If companies do fall short, we will step in and investigate, which can lead to significant fines."