The Speaking Clock has been given a new voice for the first time in 22 years.
Sara Mendes da Costa, 40, from Hove, East Sussex, officially took over the role from 77-year-old Brian Cobby at 0800 BST on Monday.
The part-time voiceover artist was named as the Speaking Clock's fourth permanent voice after a BBC Children in Need competition last year.
Ms Mendes da Costa told the BBC the organisers had been looking for "warmth and clarity and friendliness".
She said it was an "absolute honour" to have been chosen.
'Warmth and clarity'
British Telecom began the service in 1936, and Mr Cobby's voice has been heard for the past 22 years.
Sir Christopher Bland, BT's chairman, said: "It may be in its 71st year, but the BT Speaking Clock is still an important service which receives 70 million calls each year.
"The judging panel was unanimous that Sara's voice had both warmth and clarity and would be an excellent choice to succeed Brian."
She will be heard by people calling 123, except those with certain other landline and mobile operators who provide their own equivalent.