Previously unseen files reveal that successful tests on a German device manufactured in 1863 were covered up to maintain the Scot's reputation.
German research scientist Philipp Reis developed the "Telephon", which could transmit and receive speech.
It is alleged that UK businessman Sir Frank Gill was behind the cover-up. The documents were rediscovered in October by The London Science Museums curator of communications, John Liffen.
Some researchers have argued for many years that Reis beat Bell to the invention of the telephone. The archived documents seem to support their claims.
Gill was chairman of Standard Telephones and Cables (STC), the company that conducted the tests on Reis's' device.
The company was at the time bidding for a contract from the American Telephone and Telegraph Company, which evolved from the Bell Company.
Gill thought that if word got out of the test results, it would scupper STC's chances of winning the contract.
Researchers Antonio Meucci and Elisha Gray were also known to be working on speech transmission devices at the same time as Bell and Reis.
And that was a different Gill! I wonder who really invented the steam engine? Was there another Gill involved?