The communications watchdog has launched investigations into BT and Three after major outages in the summer.
The outages resulted in disruption to mobile call services, including to emergency services, across the UK, Ofcom said.
BT reported a software issue to Ofcom which disrupted call services from its EE mobile network on July 24 and 25 this summer.
The issue led to BT and EE customers being unable to make or receive calls to other networks and to emergency services.
Ofcom has already launched a probe into BT in June this year after technical faults hampered the 999 emergency call service.
Separately, Three reported an incident that hit call services on June 25, which also impacted on customers’ ability to contact emergency services.
The watchdog said it will seek to find out whether the two mobile firms failed to comply with their regulatory obligations as a result of these incidents.
A BT spokeswoman said: “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.
“We will co-operate fully with Ofcom throughout the investigation and apologise again for any issues caused by this incident.”
A spokesman for VodafoneThree said: “Three 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 co-operate fully with their investigation.”
In a statement, the regulator said: “Mobile operators are required to take appropriate and proportionate measures to identify and reduce the risks and prepare for the occurrence of anything that compromises the availability, performance or functionality of their network or service.
“Providers must also take appropriate and proportionate measures to prevent adverse effects arising from any such compromises.
“Where there is an adverse effect on the network or service, the provider must take appropriate and proportionate measures to remedy or mitigate that effect.”
