Irrationally I had expected gasps, cries – an audible pulse of panic. Instead, the windowless room they call the Pit is remarkably muted. The staff, in their green paramedic-style uniforms, are talking quietly, almost monotonously, into their headphones; more like telephonists in a bank's call centre than folk on the front line. The only sound is from a vacuum cleaner doing its work among the call-takers. There is something eerie about the stillness, because each one of the figures tapping their computer keyboard could, as I watch, be the only barrier between life and death.

"There's no protocol if someone dies on the phone," says Glen Griffiths. "You have to give what advice you can and afterwards you agonise over whether you could have done something differently and saved the life."

Griffiths has been an emergency medical dispatcher for the London Ambulance Service for two years. His is one of the voices callers hear when they dial 999 and ask for an ambulance and, at 21, he has dealt with almost every emergency conceivable. "Electrocution, explosions, a monkey attack and people getting objects stuck where they shouldn't be. A lot of the calls are about choking," he says. "It puts the heat on me; I get in a panic and try to find out where they are and suddenly they say calmly, 'It's OK, the obstruction's cleared now', and they put the phone down."

Griffiths found his way into the emergency services via Sainsbury's. "I was working in the internet shopping section dispatching deliveries and it was not challenging," he says. "My dad works for the London Ambulance Service and my mother is a nurse, so healthcare is in my blood, and the hair-raising dinner table conversations they had at home mean I'm not squeamish. I'd just never considered being qualified to work in health myself until my dad mentioned there were vacancies."

Here comes the second surprise of the evening. No university degree or healthcare background is required to be a call handler. Four weeks of training in first aid and relevant computer programs and one-to-one support for the first 10 shifts are all that is needed to equip new recruits to cope with medical emergencies. "All the medical knowledge is built into the computer systems and we have a lot of support from our supervisors," says Griffiths. "We're not clinicians."

No training can fully prepare a beginner for the stress of taking calls from casualties or bystanders in desperate situations. "I was extremely nervous the first time," he says. "My first ever call was from a lady who'd fallen over inside a friend's flat and her mobile kept cutting out. I had to try to work out where the building was from call signals.

"There are so many different types of call that until you've taken a handful you're still nervous. Even now, some can throw you into the unknown, like cases breathing difficulties, because there can be a range of causes from seizures to hanging. Then there might be a language barrier or aggression at the scene and often people haven't a clue where they are, especially if they're on a motorway."

It's a strange limbo that the medical dispatchers inhabit. The lack of windows seals them off from the outside world they are so urgently assisting. There is no indication of whether it's day or night, fine or stormy, and frequent 12-hour night shifts mean they go home to dinner as the rest of London is breakfasting. Social interaction with colleagues is conducted in half sentences, severed when a bleep through the headphones announces a new call. "It's odd when we all meet in the pub after a shift," says Griffiths. "We talk hesitantly because we keep thinking we hear a bleep in our ear."

Mostly it's not worth attempting a conversation and the dispatchers sit in silence, poised for the next alert. They talk callers through what is often one of the most desperate experiences of their lives and yet they never meet those they hand-hold and never find out what happened to them after the ambulance has collected them.

Despite this, Griffiths finds the job intensely fulfilling. "It's very satisfying to know that help has arrived for somebody," he says. "Although you often wonder what the outcome was."

Sometimes the outcome is traumatically evident: the casualty dies while still on the line. Griffiths reckons that two years has inured him to most situations, but occasional cases have reduced him to tears and a couple have forced him to take stress breaks. "There was a teenager who called because his 10-year-old sister had taken an overdose and, while I was instructing him in CPR, she vomited blood. She didn't make it," he says. "Different things can hit a nerve. If you're newly wed and someone's wife is taken ill, for instance, or if you've just had a baby and someone calls with a seriously ill child. Supervisors can always tell when someone has taken a difficult call and they send them off for a coffee break."

Pictures of lakes, flowers and seascapes dot the walls of the Pit, although their calming purpose is mocked by the staff. "We call that one Despair, that one Anguish," says a wag beside Griffiths before a call cuts him short, mid-jest.

It's extraordinary to see how Griffiths and his colleagues can switch from a casual conversation to emergency mode, without any alteration in their tone. The prompts from the computer are so familiar – address of the caller, nature of the problem, whether the casualty is conscious – that they can run through them dispassionately while speed-typing each detail on to the screen. The computer does most of the thinking, automatically dispatching an ambulance as soon as an address is typed in, deciding which of several described symptoms is the most critical and classing the case into one of four levels of urgency. A line of icons will click through to diagnostic questions for 35 different scenarios from gunshot wounds to animal bites.

But occasionally the dispatchers have to veer from the script to calm distraught callers or stabilise an aggressive situation. "You get strange reactions from people under pressure," Griffiths says. "Some are completely deadpan, almost robotic until the ambulance comes and then they break down. Some are abusive and some simply scream and you have to use repetitive persistence, asking the same question over and over, to get through to them. The most frustrating thing is when somebody rings on a mobile to say their friend has been shot, then they put the phone down. People assume you know where they are."

The two dozen staff on each shift sit in pods before two computer screens, one to interpret and act upon the clinical data and one displaying a map of London. On it, coloured circles denote ambulances en route – amber signifies help on its way, red shows the crew has arrived at the scene or are returning to hospital.

The first call of Griffiths' shift is from a carer who has found her elderly charge on the floor with a suspected stroke. Within minutes a moving orange circle shows an ambulance on its way.

"Some people here are superstitious and think that certain chairs attract more trauma," Griffiths says. "I'm a bit of a trauma magnet. The air ambulance crews get sick of me."

The biggest event he has had to cope with was the London riots: "There were a lot of assaults." Other cases stick in his mind because they are so unusual. "A man called because his hand was stuck behind a radiator and he knew his central heating was about to come on," he says. "Another man called because his wife had gone into labour [Griffiths has delivered five babies over the phone]. He was perfectly calm until he realised the baby was going to be born on his new sofa. He began shouting that he hadn't paid it off yet, and his mother-in-law had to send him from the room."

This evening's shift is proving unusually quiet. A woman calls because her brother has a nose bleed and an assault victim rings for a second time because his bleeding chin is staining his carpet. Both cases are ranked low priority by the computer. The casualties will probably get a call from a clinical team dedicated to non-urgent maladies.

"We get a lot of trivial calls – people who have seen a fox run over. But we have to understand that for them it's a frightening experience and we direct them to a more suitable source of help," he says. "Then you get people who have experienced real trauma such as the second world war, who don't like to bother us. My trainer took a call from an elderly lady who had fallen in her bathroom on a Friday evening. She kept herself alive over the weekend by drinking bath water and rang us on the Monday to ask if we were open."

It would be understandable if the continual stream of disasters that filters through Griffiths' headphones made him nervous about life's unpredictability. Instead it makes him sanguine. "It's inevitable that accidents happen and if one occurred around me I'd switch into work mode," he says. "Friends find me unsympathetic if they are unwell because I'm used to so much worse."

He has, however, no desire to put himself on the scene of a crisis and train to be an ambulance technician or paramedic. "I can deal with blood and guts," he says. "But the smell of vomit undoes me. I'd rather be safely on the end of a telephone line."

Curriculum vitae

Pay £23,861-£27,569

Hours Four 12-hour day shifts followed by three days off then four 12-hour night shifts followed by seven days off. "I can't imagine a nine-to-five job.

Work-life balance "12-hour shifts can make the weeks seem to disappear – going home and only having time to eat and wind down before sleep starts calling. On my days off I more than make up for any missed social events or downtime, doing as much as I can before getting back into my shift pattern. I share a house with six friends so there's always someone to around talk to if I've had a difficult day and there's a lot of support at work."

Best thing Seeing the difference made to such a large volume of people every shift.

Worst thing In winter, having to wake up for night shifts in the darkness, going home in the darkness and not seeing sunlight for a week!

Overtime

At the end of a gruelling shift, Glen enjoys reading books, "mostly science fiction or fantasy, never medical dramas". Glen also likes going to the cinema or watching films at home with friends, computer gaming and outdoor activities such as airsofting and paintballing. "I'm a typical 21-year-old male." Glen and his colleagues often go to the pub when they've clocked off: "we always have a laugh. It's easy to switch off when you're on a break and a lot of us have a dark sense of humour, which I guess you need in this job."