The Scottish-born Bell worked in London with his father, Melville Bell, who developed Visible Speech, a written system used to teach speaking to the deaf. In the 1870s, the Bells moved to Boston, Massachusetts, where the younger Bell found work as a teacher at the Pemberton Avenue School for the Deaf. He later married one of his students, Mabel Hubbard.

While in Boston, Bell became very interested in the possibility of transmitting speech over wires. Samuel F.B. Morse’s invention of the telegraph in 1843 had made nearly instantaneous communication possible between two distant points. The drawback of the telegraph, however, was that it still required hand-delivery of messages between telegraph stations and recipients, and only one message could be transmitted at a time. Bell wanted to improve on this by creating a “harmonic telegraph,” a device that combined aspects of the telegraph and record player to allow individuals to speak to each other from a distance.

With the help of Thomas A. Watson, a Boston machine shop employee, Bell developed a prototype. In this first telephone, sound waves caused an electric current to vary in intensity and frequency, causing a thin, soft iron plate–called the diaphragm–to vibrate. These vibrations were transferred magnetically to another wire connected to a diaphragm in another, distant instrument. When that diaphragm vibrated, the original sound would be replicated in the ear of the receiving instrument.

On February 14, 1876, Alexander Graham Bell applied for a U.S. patent for the telephone. On March 7, 1876, Bell was awarded U.S. patent 174,465. This patent is often referred to as the most valuable ever issued by the U.S. Patent Office, as it described not only the telephone instrument but also the concept of a telephone system.

According to britannica.com & history.com