A voice can be considered an instrument, I think. Though I cannot claim that my voice is any instrument as I cannot sing very well.
Yes, classical vocalists like to think of their voice as an instrument, both the easiest and hardest instrument to play. Like someone said, sure anyone can sing, but its incredibly difficult to gain full control of you voice. Operatic singers, who are unquestionably the best vocalists in the world, don't even begin to master their voices until their late 30s.
That being said, I'm a fairly good bass, on a good day I can hit a nice low C, which would make me a basso profundo.
I'm singing in my college's top chorale, out of the 3 total choirs. I also am actively engaged in lessons. Music is a big deal here at Valpo, my conductor directs a Bach Institute.
Other than that, I play decent metal rhythm guitar, passable acoustic, and novice classical guitar. I can do fairly well on bass too. And I had enough piano lessons that I can play basic chords and plunk out my vocal line.