The first voice to be heard over a telephone was that of the inventor of the telephone, Alexander Graham Bell. His voice was first heard by his assistant, Thomas Watson, in their laboratory in March of 1876.
At that time, Bell was working on the telephone and called out to his assistant, "Mr. Watson, please come here. I want you." At that time, Watson was not actually on the same floor of the building. But it happened that he was near to a telephone receiver and heard Bell's voice through the device.
So, the first voice heard by telephone was that of Alexander Graham Bell.