Katie Vallis is three hours into a 12-hour shift answering 999 calls at the London Ambulance Service's emergency operations centre in Waterloo when her 27th call of the day is patched through to her headset.

"I think I've dislocated my shoulder," barks a 30-year-old male caller.

"How?"

"Dunno, really. I was just on my sofa. I think I must have been lying on it wrong. I need an ambulance."

Without even glancing at her keyboard, Katie, 20, calmly starts to take the caller's details and type them into the system. As she enters the address, a detailed map of the caller's area in south-west London flashes up on her screen. She points to the nearest A&E department, less than a mile from the caller's home.

"I don't know why he doesn't just walk himself down to A&E," she says to me with her hand over the microphone, before explaining that, no matter what she thinks of a caller's predicament, all calls are logged and assessed. An "allocator" on the other side of the huge operations room will decide whether an ambulance should be diverted to the address and, if so, which one.

Another 999 call comes through - this time an "RTA" (road traffic accident) in Islington, north London, involving a motorcyclist, who is unconscious. Katie receives this kind of call every few minutes.

Months earlier, I had also made a 999 call for an ambulance when my wife went into the advanced stages of labour at home. Katie was the dispatcher - they dislike being called "operators" - who answered that day and, with the midwives delayed in traffic, she talked me through how to deliver my son over the course of the 11-minute call. Naturally, this formed a bond between us - Katie was still new to the job and it was her first-ever "BBA" (born before arrival) - and when we finally met face-to-face, I took up Katie's offer to spend a shift listening in to the huge variety of calls the emergency operations centre deals with each day.

This is the world's busiest ambulance control room, although you wouldn't know it from the building's characterless facade. Once through security, you can spot clues about its importance to the capital's wellbeing. Plaques and award certificates line the corridor, but it is the signed photographs praising the building's occupants that catch the eye - Tony Blair, Queen (the band rather than the monarch) and footballer Rio Ferdinand, to name a few. Many make direct reference to the LAS's most testing day in living memory - July 7 2005.

In the main control room - a second, smaller room for full-scale emergencies lies empty on the opposite side of the corridor - I am shown to Katie's desk. She sits with around 25 other dispatchers, with a similar number of allocators on a platform to the right. Allocators are charged with managing the movements of the service's 388 ambulances and 142 rapid-response vehicles.

This room handles medical emergency call-outs across the whole of Greater London, a 620 square-mile area with a population of more than seven million. In 2006/7, the LAS handled 1.2m calls and attended 865,000 incidents. (During a busy period, the control centre receives about 250 calls an hour.) It estimates that every time a call to the service requires a response, it costs the taxpayer £165.

I have arrived mid-afternoon on a cold, clear winter's day. Sitting beside Katie, I ask whether she can judge by the time of day, or even the weather, if she is going to be in for a busy shift.

"Rush hour is just getting going now, with the school run in full flow," she says. "This can be one of the busiest times. When people are waking up can be busy, too, as they might have discovered that a relative has died in their sleep. New Year's Eve is the busiest night of the year, plus the Christmas party season, when the volume of calls rises by about 15%. Ice and wet weather are always bad for accidents. Also, if it is really hot, with people out drinking all day, we get a lot of calls. You also get the elderly collapsing in the heat."

The LAS estimates that nearly 5% of all 999 calls it receives concern alcohol-related incidents, although the figure rises to 14% between 12am and 1am on Saturday nights. To date, the relaxation of licensing hours hasn't caused a discernable decrease in the number of alcohol-related "disorder flashpoint" calls, which spike between 11pm and 2am on Friday and Saturday nights. In total, alcohol-related 999 calls have risen by 12% in the past two years. Occasionally, the LAS experiences a perfect storm: the weekend that England was knocked out of the World Cup in 2006 also coincided with a heatwave, and the service handled 9,500 calls in two days.

In quick succession, three calls come in. First, a care worker at a nursing home says an 87-year-old woman resident has fallen to the ground and is fitting. Next, a 37-year-old man has fallen over and cut his eye after the bus he was on braked suddenly. Then a young woman calls from her mobile to say she thinks a teenage boy has been stabbed outside a shop in north London.

Katie says these are all routine calls for her and she can often deal with them in a matter of seconds. With the suspected stabbing, she notices other dispatchers in the control room around her also taking calls from witnesses at the scene. She logs into her system a few minutes later to follow the progress of the incident and notices that the air ambulance has been stood down because paramedics are at the scene.

This call will remain etched into my memory for days afterwards, but Katie says she usually forgets them within a few hours, such is the pace and volume of calls she handles. The ones that do stick in her mind involve sexual assaults or children who have died.

"We get stress breaks and we can also ask for counselling if we need it," she says. "So far, I have only experienced two people dying during the call. We can also get suicide-related calls, but many are classic calls for help and we just try to distract them with a chat."

Their frontline position means that the dispatchers deal with more than medical emergencies: they also report suspected cases of abuse of children and vulnerable adults. In 2006/7, 1,896 adults and 574 children were recommended for "further investigation" by the LAS.

Each call is given a priority code and this determines the response time. A "red" call is the most medically pressing; for example, a heart attack or serious head injury. Medical teams must aim to be on the scene within eight minutes of the moment the dispatcher logs the details. The target for "amber" calls ("serious but not immediately life-threatening", such as strokes and back injuries) is 19 minutes and for "green" calls ("neither serious nor life-threatening", such as hospital-to-hospital transfers) it is an hour. From April, new targets mean that the clock will start ticking from the moment the call comes in, which will, in effect, shave two minutes off the current response limits. (Katie's tip when calling in an emergency is, if possible, to dial 999 from a landline as the location of the phone immediately flashes up on the dispatcher's screen. Calling from a mobile can add seconds, or even minutes, as the dispatcher must first establish the location of the caller.)

"Sometimes I won't get any all day," says Katie. "Sometimes, four to five a day will come in." The LAS estimates that only one in every 10 calls involves someone in a potentially life-threatening situation.

Initially, the control room computer will grade the call by assessing the data typed in by the dispatcher. The four main factors are "chest pain", "breathing", "haemorrhaging" and "consciousness". If any of these are entered as a problem by the dispatcher then the call will automatically be graded as a "red".

It is not a foolproof system. A drunk who has passed out will be classified "red" if they are deemed to be unconscious. "This annoys the ambulance crews," says Katie, "but there's nothing you can do about it."

Katie adds that some "frequent fliers" - the nickname for persistent callers - have got wise to this system and now know that if they mention "chest pains" an ambulance will be dispatched automatically. Many "frequent fliers" are well known to dispatchers and they will often realise who is calling before answering because their number will be recognised by the system and trigger an alert on screen. This is also true for anyone with a history of violence, as the crew in attendance will require a police escort. In February, a woman who had called 999 for an ambulance more than 1,000 times in the past three years received a 28-day suspended prison sentence. She was also prohibited from making "false and unnecessary calls".

As Katie's shift draws to a close, the calls continue: there's another motorbike accident; a 35-year-old woman calling from a tube station says she got off the train after getting "irregular heartbeats"; a young heroin addict says she thinks she's pregnant and is worried because she's started bleeding and has just "seen a rat while eating a melon in the kitchen".

Most shifts conclude with a team drink at the local pub, but as she is facing a run of four night shifts Katie says she needs an early night. Before I go, I ask her how long she expects to keep doing such a high-adrenaline, emotionally draining job.

"I love it, but my real aim is to go out on the road," she says. "First, though, I have to go to university and study paramedic science."

Last summer, Katie was working at her local Woolworths when a friend persuaded her to download an application at the LAS website. Like thousands of others who have been lucky enough to have her answer their 999 call, I can't thank her enough.