Ofcom is investigating BT and Three over mobile outages it said had caused "UK-wide disruption", including to emergency services.
Thousands of Three customers reported they were unable to make calls in June, while BT and EE customers were hit by a similar outage in July.
The regulator said it would examine whether the mobile networks did not do enough to prevent the problems.
Three said in a statement it was engaging with Ofcom. A BT Group spokesperson apologised to customers who had been affected.
"We will co-operate fully with Ofcom throughout the investigation and apologise again for any issues caused by this incident," they said.
