Phone books going back to 1880 have been put online, making it easier to trace family histories.
Even Buckingham Palace was in the phone book
The resource at ancestry.co.uk will also help in finding out more about previous residents of people's homes.
Some famous names who could have been phoned in their day include Winston Churchill in 1925, Laurence Olivier in 1930 and even spy Kim Philby in 1943.
Ancestry.co.uk describes the project with BT as an "important archive for family and social historians".
Josh Hanna, managing director of the website, said: "The British phone books, from 1880, are a fascinating, important addition to our online records and provide family and social historians with unique 20th Century information which has been very difficult to find up until now."
BT Archives head of heritage David Hay added the books had always provided a "unique snapshot" of communities in the UK.
Other well-known numbers in the books include those of telephone inventor Alexander Graham Bell in 1880, Dracula author Bram Stoker in 1910 and escapologist Harry Houdini in 1911.
Sigmund Freud, Benjamin Britten, Noel Coward, Ian Fleming and Sir Oswald Mosley were all listed too.
Initially the 430 books for Greater London containing more than 72 million names will be available on the website.
All the UK phone books up to BT privatisation in 1984 are public records and feature more than 250 million entries.