The communications watchdog has launched an investigation into BT after technical faults hampered the 999 emergency call service on Sunday.
Ofcom said it would look into the incident to determine whether the telecoms giant failed to comply with its regulatory duties.
Emergency calls in some areas of the UK were hit by outages from around 8.30am, which prevented calls from connecting to an operator.
BT, which manages the 999 phone network, switched to a back-up system at around 10am but some services continued to report delays in calls being received.
The Metropolitan Police said the back-up system was not as effective at locating callers.
BT apologized “sincerely” for the UK-wide disruption and said the issues were resolved by Monday evening.
Under Ofcom’s rules, BT and other network providers must take all necessary measures to ensure uninterrupted access to emergency organizations as part of the call services they offer.
They must also do what they can to prevent and prepare for systems breaking down or technical faults.
“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”, said Ofcom.
“Our 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.”
BT said it is undertaking its own investigation into the incident alongside Ofcom’s probe.
A spokesman for BT Group said: “We’re nearing the end of a full, internal investigation and expect to share the findings with Government, the emergency services and Ofcom (with whom we are in regular contact) by Thursday.
“This will examine the technical aspects of what triggered Sunday’s incident, the process of moving over to the back-up system, and the timings of communications to the emergency services, Ofcom and Government.
“In the interests of transparency, we will share the key findings publicly at the same time, subject to the removal of any information that remains confidential for critical national infrastructure.”