For historical allocation purpose, the first transmission of the spoken words on wires was held by Alexander Graham Bell and his assistant Thomaz Watson on June 2nd, 1875 when conducting experiments on the transmissions of a number of telegraph messages to be sent simultaneously over a single wire by means of

Fig. 344 - The first Bell’s telephone system .

interrupted tones of different frequencies as known as harmonic telegraph. Fig 344
The harmonic telegraph consisted of an electromagnet comprising tuned vibrating reeds at the transmitting end and similar ones at the receiving end.
From the theory only the reed tuned in the corresponding frequency should receive a signal sent by mean of a vibrating reed. Thus, by using different frequencies it should be possible to send several telegraphic messages simultaneously over a single wire.
While starting one of the instruments as the transmitter, Watson pressed too hard the fixing screw contact point against the reed and so it stopped in vibrating accordingly. Thus, in order to readjsut it he tapped the reeds gently with the fingers and suddenly the generated vibrating sound could be heard in the receiving device held by Bell located in a nearby room.
Through this historical experiment Bell was aware that the vibrating sound could be transmitted on wires, which certainly was the foundation for building of the first telephone device. Originally it consisted of a wooden frame on which was assembled the electromagnet provided with a steel latch. In one of the latch’s free ends was mounted a tightly stretched membrane. When speaking over the assembling mouthpiece, the sound waves force the membrane to follow the vibration of the voice and so generate voice-shaped electric undulation.
Soon the first telephone was tested using a harmonic telegraph as receiver for listening. Even considering that on the course of the experiment it was possible to hear only feeble noisy produced by the spoken words, certainly it arose the feeling that the inventor was on the right track.
In March 7, 1876, U.S. patent n° 174,465, was issued to Alexander Graham Bell for his telephone system. Therefore, it was three days later, in March 10, 1876, that finally the inventor succeeded to transmit the first complete intelligible message by by mean of electricity.