BT is being investigated by a UK regulator following widespread distortion to emergency call services on Sunday.
Ofcom said its investigation will “seek to establish the facts surrounding the incident and examine whether there are reasonable grounds to believe that BT has failed to comply with its regulatory obligations.”
Supposed “technical faults” caused people not to call the police, ambulances, and fire services for more than an hour on Sunday morning.
The issue was spotted shortly after 8.30am on Sunday morning, but at 9.52am, BT announced that its backup platform was “now working” and confirmed people should call 999 “as usual”.
Ofcom said its rules require BT and other network providers to take all necessary measures to ensure there is no uninterrupted access to emergency services.
Providers are also required to take measures to ensure the fullest possible availability of calls and internet in case of a network breakdown.
BT and other providers are also required to take “appropriate and proportionate” measures to identify and reduce the risk of anything that could compromise their service.
“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,” Ofcom added.