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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 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.
Meanwhile, Three said it had “experienced disruption to voice services following an exceptional spike in network traffic triggered by a third-party software configuration change”.
“Since the outage, we have engaged openly with Ofcom and will continue to cooperate fully with their investigation,” it 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 importance of connectivity cannot be underestimated,” said telecoms analyst Paolo Pescatore.
“We all demand a robust and reliable connection at home and out and about.”
He said outages can occur despite “significant efforts” to prevent them – but said there must be “a straightforward process to identify the issue and to learn lessons so it does not happen again”.
Previous problems
The incidents at the heart of Ofcom’s investigation saw thousands of customers 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.
