Reis was born in Gelnhausen, Germany, as son to a Jewish baker. His parents died early, and he was raised by his grandmother. He studied mathematics and natural sciences on his own. In 1860, he constructed the first prototype of a telephone, covering a distance of 100 m. He did not manage to get people interested in his invention, however, and is was largely forgotten, except by Alexander Graham Bell, who demonstrated and patented an improved version of Reis' apparatus in 1876.
Documents in the London Science Museum show, that, in 1947, engineers from the British firm Standard Telephones and Cables (STC) found, that Reis's device dating from 1863 could transmit and "reproduce speech of good quality but of low efficiency".
Sir Frank Gill, then chairman of STC, ordered, that the tests be kept secret, as STC was negotiating with the AT&T, which had evolved from the Bell Company of Alexander Graham Bell. Bell was generally accepted to have invented the telephone, and Gill thought that evidence to the contrary might disrupt the negotiations.
Besides Reis and Bell, Antonio Meucci and Elisha Gray both invented similar devices. Meucci was officially recognized as inventor of the telephone on September 25, 2001, by the US Congress.