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Many 999 calls are not connecting due to a technical fault affecting a number of police forces, the Metropolitan Police said.
It has urged people to call 101 in an emergency until further notice.
The public are being advised to save non-emergency calls for later.
Scottish Fire and Rescue, Police Scotland, West Midlands Police, Devon and Cornwall Police, Norfolk Police and South Yorkshire Police are also experiencing 999 issues.
Problems with the service emerged at about 09:00 BST on Sunday.
BT runs the 999 number for emergency services and is responsible for answering and directing calls.
The Met Police said it was not aware of the problem being due to hacking.
A BT spokesperson told the BBC that the situation was “fast-moving as we fix the problem”.
At about 10:00 BST on Sunday some emergency services said emergency call services were being restored.
BT said 999 calls could now be taken following the earlier fault.
Cheshire Fire and Rescue Service said services were “now being resumed so in an emergency please dial 999 in the first instance”.
Suffolk Police warned that the “system may not be working to full capacity so please only call 999 in an emergency”.
Emergency services experiencing technical problems include:
- Avon and Somerset Police
- Cheshire Fire and Rescue Service
- Cumbria Police
- Derbyshire Police
- East England Ambulance Service
- Essex Fire Service
- Devon and Somerset Fire and Rescue Services
- Gloucestershire Fire Service
- Gloucestershire CC
- Kent Police
- London Ambulance Service
- North West Ambulance Service
- North Yorkshire Police
- Northumbria Police
- South Wales Police Cardiff
- Tyne and Wear Fire and Rescue Service
- West Yorkshire Fire and Rescue
- West Midlands Ambulance Service
- West Midlands Police
- Wiltshire Police
- Yorkshire Ambulance