2 hours ago
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Liv McMahonTechnology reporter
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 has not yet responded to a request for comment - but 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.
According to Ofcom, firms must take proper action to identify risks and prepare for "anything that compromises the availability, performance or functionality of their network or service".
It said providers must also prevent "adverse effects arising from any such compromises" - saying where this happens, they must take steps to mitigate them.
The incidents at the heart of its investigation saw thousands across BT and Three networks report problems with their mobile service.
At the time, Three told customers complaining of issues making and receiving calls on 25 June it was experiencing "an issue affecting voice services".
This was not isolated to its own network - it also caused problems for customers on networks that piggyback off of Three, such as ID Mobile.
A month later, EE and BT customers complained of similar issues.
A government spokesperson said at the time "communications providers have statutory obligations to ensure their networks and services are appropriately resilient".
Operators have previously faced scrutiny over outages or issues affecting peoples' ability to make calls or contact emergency services.
BT was fined £17.5m in July 2024 for a "catastrophic failure" of its emergency call handling service which led to thousands of 999 calls not being connected.
Three was ordered to pay £1.9m in 2017 after Ofcom found it could have prevented an issue that caused a loss of service for customers a year prior.
It has since merged with Vodafone to form the UK's biggest mobile network, with 27 million customers.
