The idea of creating a clock that would tell the time accurately over telephone lines dates back to the 19th century. The first such service was provided for residents of San Francisco, who could connect to an observatory clock, listen for a minute, and then set their clock to the current time - assuming they could correctly interpret the series of clicks and buzzes they heard on the other end.

A talking clock was first introduced, again in America, in 1927. The Frence adopted the idea with a Parisian speaking clock starting service in 1933. They were followed by the Dutch, who instituted the service in the Hague the following year, and the year after that by the Swiss. It was not until 1936 that the idea was taken up in England, where the service was at first limited to the London area, although by 1942 it had become a nationwide institution.

Originally the British service was known as TIM, this being the combination of letters used to dial the speaking clock on one of the early alphanumeric telephone dials. The now familiar number 123 only became standardised in the early 1990s. The clock mechanism consisted of an array of four glass disks which were read by a photoelectric cell. Each disk contained recordings of the various sections of the time message, for example: 'At the third stroke it will be five twenty-five and twenty seconds'. A series of three pips then marked the time.

The voice of the speaking clock was provided by Ethel Cain, one of 15,000 female telephonists who entered a competition to find the best voice for the job. She was awarded the princely sum of ten guineas for her efforts, and her voice continued to resound across the telephone wires for the next 27 years.

In 1963 it was deemed time for an update, and another woman - Pat Simmons, a supervisor at a London telephone exchange - was selected to voice the talking clock. In 1984 Miss Simmons' mellifluous but slightly reedy tones were replaced by the voice of a man, Brian Cobby. A former actor who had changed careers to become assistant supervisor at the Withdean telephone exchange in Brighton, Mr Cobby's previous claim to fame was as the voice that spoke the immortal lines '5 - 4 - 3 - 2 - 1 ... Thunderbirds are go!' at the start of the popular children's TV series.

The wording used in the speaking clock announcement remained the same until 1986, when BT executives recognised the sponsorship potential of the service. Since then Brian Cobby has spoken the phrase 'At the third stroke the time sponsored by Accurist will be ...'

Miss Cain, Miss Simmons and Mr Cobby remained the only voices to have been heard on the speaking clock's number until March 2003, when comedian Lenny Henry recorded a special version of the clock's announcement in support of the Comic Relief charity.