An investigation has been launched into BT after 999 stopped working for dozens of emergency services on Sunday morning.
Watchdog Ofcom said it was looking into the UK-wide issue that meant many callers couldn’t get through on the usual number.
Police, fire and ambulance services had to tell people to call 101 – normally used for non-emergencies – and local numbers instead.
More than 50 regions and counties reported problems.
The disruption lasted a few hours before 999 was restored. BT said it used a back-up system to get things going again.
Ofcom will look into whether the firm failed to comply with its obligations and took appropriate measures to reduce the impact.
“Our rules require BT and other providers to take all necessary measures to ensure uninterrupted access to emergency organisations as part of any call services offered,” said Ofcom.
Hours after things got back to normal, Met Police said it was struggling to deal with record numbers of emergency calls.
The London force warned it could mean “we don’t get to someone in danger in time” and suggested it was partly down to people dialling 999 accidentally.
It urged people with Android phones to prevent “pocket dials” by disabling the SOS option in their settings.
An update to many devices – meaning 999 is called when the power button is pressed five times – has been blamed for an increase in accidental calls.