At the third stroke, the speaking clock will be 75 years old, precisely

And, to mark the occasion, some other familiar voices are stepping out of their recording booths

You hear their voices every day, but you could walk past them on the street without a second glance. They are the voice artists: at once the most recognisable yet most anonymous people in Britain.

Next week, BT's Speaking Clock is 75 years old. To mark the anniversary, Britain's best-known voices are showing their faces and talking about the little-known lifestyle of the voiceover artist.

Sara Mendes da Costa is the fourth and latest voice of the clock, which began telling us the time in 1936. She got the job after a public audition in 2006. "They were looking for someone with a trustworthy, friendly, open and warm voice, and the judges said I fit the bill," she says. Since recording the clock, Ms da Costa makes a living recording at home.

In recent years, voice work has become big business. Every advert, telephone holding message and public service announcement requires someone to sit in a booth and reel off the lines.

The home recording booth, often sound-proofed with blankets and bedding, is the ultimate tool of the trade. Emma Clarke is one of the biggest names in the business. On a normal day, she records a voiceover every 20 minutes from her home studio. You will almost certainly have heard her somewhere – most likely on the London Underground; she's the one saying "Mind the gap". Unfortunately, she can't stand the sound of her own voice, so she avoids the Tube at all costs.

Not being able to escape their own voice is a problem for many in the industry. Terry Green's is the voice that says "Cashier No 3 please!" in 8,500 post offices, banks and shops nationwide. It

is estimated that Terry is heard 30 million times every month.

There are, of course, big opportunities in showbusiness for voiceover artists, but even they remain anonymous to most. One of Britain's most recognisable voices thanks to his work on The X Factor, is Peter Dickson's, whose bombastic delivery has taken him from jobbing reporter on local radio all the way to Hollywood. "A show like The X Factor demanded a really big voice, and I think I delivered," he says, in his slightly more subdued regular voice. He's now trying to crack the market for movie trailer voiceovers in Los Angeles.

Then there's the unmistakable "voice of the balls", on the National Lottery, Alan Dedicoat. Though famous for his excitable tones, he admits that there are days when he's just not "in the mood". "Sometimes you have to turn it on very quickly. You could be breaking some very big news for someone, somewhere – you don't want to sound miserable," he says.

Veteran voice artists can become icons without ever showing their faces. "Not a week goes by that someone doesn't hear my voice and say – 'My God, you're Mr Benn!'" says actor Ray Brooks, narrator of the 1970s children's TV classic Mr Benn. "I only did 14 episodes in all, but the cultural impact is extraordinary."

Most beloved of all, perhaps, is James Alexander Gordon, the 75-year-old broadcaster known to millions of football fans as the voice of Radio 5's classified football results on Saturdays.

"After nearly 40 years, I still get a kick out of it," he said. "People feel connected to you because of your voice, and that's a real privilege."