Emergency 999 calls failed to connect across the UK this morning amid technical issues with the phone lines.
Brits were urged to call 101 in an emergency for police and fire, or 111 in a medical emergency, after BT reported a "system failure".
Several police forces and other emergency services have since confirmed they are able to receive 999 calls again.
Some said they were experiencing the "residual impact" of the national fault and urged people to use 999 only in a genuine emergency.
A spokesperson for BT said: "Earlier today we experienced a technical fault with the 999 service. We are sincerely sorry for the distress this has caused.
"There was a short period of time as we switched over to the back-up system during which calls could not get through. In this instance, we are still able to see who is trying to reach the operator, and our handlers reconnect calls as a matter of urgency.
"Our back-up platform took calls throughout most of Sunday, with people able to call 999 as usual.
"The primary 999 service was restored earlier this evening and we are no longer relying on the back-up system.
"We are monitoring the service, and we continue to work hard to determine the root cause and the impact this has had.”
Earlier, the Metropolitan Police advised people to phone 101 in an emergency.
A force spokesperson said: "Due to a technical fault that is impacting a number of police forces, many 999 calls are not connecting.
"Until further notice, anyone in London who requires the police in an emergency is asked to call 101.
"Please only call in an emergency and please wait until later to make any 101 non-emergency calls."
Northamptonshire Police said the problem was "nationwide", although it was not clear how many emergency services were affected.
Other forces reporting issues included Greater Manchester Police, Hampshire Police, Essex Police, Bedfordshire Police, West Yorkshire Police, West Midlands Police, Norfolk Police, South Wales Police and Police Scotland, among others.
Fire services, including Bedfordshire and Leicestershire, also warned of the fault, as did ambulance services such as the East of England Ambulance Service.
The NHS earlier told Brits to phone 111 in a medical emergency.
A spokesperson said: "There is an issue affecting the national 999 call system.
"BT is working to resolve this issue as soon as possible but, in the meantime, if you are unable to contact 999 in a medical emergency, please call 111."
Some services reported that the problems had been resolved at around 9.30am.
