The UK communications regulator has opened investigations into BT and Three following major network outages over the summer.
Ofcom said the incidents caused widespread disruption to mobile call services across the UK, including access to emergency services.
BT notified Ofcom of a software fault that affected call services on its EE mobile network on July 24 and 25. The issue meant BT and EE customers were unable to make or receive calls to other networks, including calls to the emergency services.
The watchdog had already launched a separate investigation into BT in June after technical problems disrupted the 999 emergency call service.
In a separate incident, Three reported an outage on June 25 that also affected call services and prevented some customers from contacting emergency services.
Ofcom said it will examine whether either operator failed to meet their regulatory obligations as a result of the outages.
“We are aware that Ofcom has begun an investigation into the technical fault impacting voice calls on our network on July 24 and 25 2025,” a BT spokeswoman said.
“We will co-operate fully with Ofcom throughout the investigation and apologise again for any issues caused by this incident,” she added.
