Was "Colossus: the Forbin Project" the first time we heard what is now considered the "Cylon" voice?
If you mean a voice filtered through a vocoder, that effect was used 3 years before Colossus in the theme song to Gerry Anderson's Captain Scarlet and the Mysterons. A similar effect was used to create Cyberman voices in Doctor Who episodes from 1966-69, although it involved placing a device in the mouth to generate a pure tone that the actors would modulate with their lips and tongues like normal speech.