The first voice radio-telephone call
On February 22,1880 Alexander Graham Bell and his cousin Charles Bell communicated over the Photophone, a remarkable invention conceived of by Bell and executed by Sumner Tainter. [Grosvenor] This device transmitted voice over a light beam. A person's voice projected through a glass test tube toward a thin mirror which acted as a transmitter. Acoustical vibrations caused by the voice produced like or sympathetic vibrations in the mirror.
Sunlight was directed onto the mirror, where the vibrations were captured by a parabolic dish. The dish focused the light on a photo-sensitive selenium cell, in circuit with a telephone. The electrical resistance of the selenium changed as the strength of the received light changed, varying the current flowing through the circuit. The telephone's receiver then changed these flucuating currents into speech.
Although not related to the mobile telephony of today, Bell's experimenting was a first: radiated electromagnetic waves had carried the human voice. Despite Bell's brilliant achievement, optical transmission had obvious drawbacks, only now being overcome by firms like TeraBeam. Most later inventors concentrated instead on transmitting in the radio bands, with the period from 1880 to 1900 being one of tremendous technological innovation.
For ruminations on the Photophone and how to improve it go here: http://jefferson.village.virginia.edu/~meg3c/id/id_edin/ph/ph1.html
For a fascinating look at how ham radio operators can communicate optically click here
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Resources
[Grosvenor] Grosvenor, Edwin S. and Morgan Wesson. Alexander Graham Bell : The Life and Times of the Man Who Invented the Telephone Abrams, New York (1997) p.102.
Editor's note: The Photophone photograph that accompanies the text is from Grosvenor's excellent book. I never take pictures from books still in print but I have been unable to find any accurate picture of the Photophone on the net. I will immediately remove this image once I do.