The London Ambulance Service has become the first in the UK to use new technology to pinpoint the location of mobile phone callers.
Ambulances should now respond more quickly
Officials say it will help them to respond to 999 calls much more quickly.
The London Ambulance Service receives around 750 calls from mobile phones each day.
However, in many cases callers are unfamiliar with their surroundings and are unable to say precisely where help is needed.
The new technology enables operators at the service's control room to see where the mobile phone user is calling from.
"When you make a 999 call you are connected to a BT operator," says Quentin Armitage, deputy director of technology at the London Ambulance Service.
"At the same time, the mobile phone operator provides information on the cell that you are in or what mast you are connected to to a BT database.
"When that call is passed through to us we are then able to look up that information in the BT database and we can display the area of coverage, the circle on a map for our call centre so they can what area the caller is in."
Philip Selwood, director of the London Ambulance Service, said the technology would benefit patients.
"It is reducing the time it takes to confirm the location of mobile phone callers which not only helps to reduce the stress on them but also enables us to dispatch the ambulances more quickly."
Ofcom, which regulates the phone industry, welcomed the move.
"It represents a major step forward in helping the emergency services to respond to calls from mobile users as quickly as possible," said Peter Walker, a senior adviser at Ofcom.
The technology was developed by Ofcom in association with the UK's mobile phone operators, BT and Cable and Wireless.