Source: Sharecast
Thousands of Three customers found themselves unable to make any calls, including to the emergency services, on 25 June, while customers of both BT and its mobile network operator EE suffered similar problems on 24 and 25 July.
In BT’s case, a software issue was blamed for the UK-wide disruption to mobile call services interconnecting to and from the EE network. As a result, BT and EE customers were unable to make or receive mobile calls to other networks and emergency services. Both BT and Three informed Ofcom the incidents.
Launching the probe, Ofcom noted that mobile operators were required to take "appropriate and proportionate" measures to identify and reduce any risks to the functionality of their networks.
It continued: "Our investigations will seek to establish the facts surrounding these incidents and asses there are reasonable grounds to believe that BT and Three have failed to comply with their regulatory obligations."
Three said: "Since the outage, we have engaged openly with Ofcom and will continue to cooperate fully with their investigation."
BT said: "We will co-operate fully with Ofcom throughout the investigation and apologise again for any issues caused by this incident."
BT was fined £17.5m in 2024 after a network fault lasting 10 hours in June 2023 resulted in 14,000 calls to emergency services failing to connect. Three has also previously fined by Ofcom, paying £1.9bn following a loss of service in 2016.
Three is now owned by VodafoneThree, after Vodafone and CK Hutchison merged their UK operations earlier this year in an $19bn deal.