Hannah Foster was murdered after her 999 call was judged to be accidental and cut off. But how do police decide if an emergency call is a misdial or a desperate last plea for help?
The number 999 is a lifeline for those in distress. Probably every adult knows its purpose and, when faced with an emergency, even toddlers have been able to dial these crucial three digits.
But, as the case of murdered teenager Hannah Foster demonstrates, there are times when emergency calls can be cut off by the operator at the other end.
When Hannah was abducted in March 2003, she was quick-witted enough to surreptitiously make a 999 call on her mobile. Unable to speak into the phone, she nevertheless hoped the operator would become suspicious of the conversation she was having with her abductor, Maninder Pal Singh Kohli.
Unfortunately, Hannah's attempts failed to navigate a call handling system called Silent Solutions, which deals with silent calls made to the 999 number, and she was cut off moments later.
Kohli went on to rape and murder her, and has now been jailed for life following a trial at Winchester Crown Court.
Accidental 999 calls happen all the time, but how do operators know when a silent call can actually be a real call for help?
BT receives 30 million emergency calls a year - either to 999 or 112, the European emergency services number, which works in all European Union countries. There are strict procedures for handling such calls, set out in a code of practice between telecoms operators and the emergency services.
Calls made to these numbers usually result in an emergency authority (EA) - fire, police, ambulance - request being made, where someone asks the operator for assistance.
But several million calls a year do not follow this pattern, and the emergency number is dialled but no-one actually speaks to make a request. This is what happened in Hannah's case.
The code of practice says in the "overwhelming majority of cases" these calls are customer misdials, such as a phone keypad activated in a bag or pocket, or even children playing. But it acknowledges "there is always a possibility of it being a genuine caller who cannot speak". It also says "very large numbers" of accidental 999/112 calls are received from mobile phones.
In 2001 the Metropolitan Police introduced a country-wide system called Silent Solutions for dealing with the growing number of silent accidental 999 calls. When a call is made, operators from police forces around the UK attempt to obtain a response by asking a series of questions.
For example, they might ask "which service is required?" and "if you cannot speak but need help please tap the handset screen".
But when nothing apart from general noise can be heard, and there is no speech, it is thought there is a "negligible chance" of the call being genuine. The operator can then end the call.
Where there is no response but there are background voices, the code of practice says the operator cannot decide whether an Emergency Authority request is needed from the police.
In cases such as these, the call is then connected to an automated police voice response system at the Met's Central Communication Command, which asks the caller to press five twice if help is required. If 5-5 is pressed, an immediate connection with the police is made.
And in any cases where suspicious noises are heard, the operator can override all these procedures and simply connect the call directly to a police emergency authority control room.
"Most silent accidental 999 calls contain background noise, which is usually very difficult to hear or understand," says a spokesman for the Met. "It was only after Hannah's 999 call was forensically examined during the subsequent investigation when the audio had been expertly enhanced that it was possible to capture the voice of her abductor and the conversation they had."
Silent calls are now common. Between July 2001, when the Silent Solutions system was introduced, and September 2008, there have been more than 40 million such calls - averaging about 5.5 million a year. The overwhelming majority of these are unintended 999 calls, says a Met spokesman.
"Since 2004, on average each year, we have received about 47,000 silent calls during which the caller pressed '55' and therefore indicated they needed an emergency service, which means that only about 0.9% of the silent calls we receive are intentional."
Yet Hannah's tragic case is not a one-off. In 2005 Farah Noor Adams was raped and murdered in Glasgow. She had made a number of silent calls when she spotted she was being stalked by her eventual killer. But they too were cut off by operators when she failed to respond.
But there are occasions where silent calls do work. One woman who was attacked at her home in Kensington, London, managed to make a silent 999 call. This led to the police tracing her call and visiting her home, where they captured her assailant. He was eventually convicted in 1991 of rape, possession of a firearm and attempted murder.
And emergency services in some parts of the country now respond to texts sent to 80999. This allows messages to be sent directly to the police control room. The controller then calls for assistance as required from any of the emergency services.
The 80999 number is currently used in the South West, in Hampshire, the Isle of Wight, Dorset, Wiltshire, Avon and Somerset and Devon and Cornwall, and can be used at any time.
However, this is provided as a specific access service for people who cannot use voice telephony, and is not considered as an additional service for general contact. Several other police forces already run emergency text services, but they require people to register their details and use a standard 11-digit mobile number.
The Home Office plans to introduce a National Emergency Text number, which will eventually replace these systems.