At work I'm often asked to speak for them in the P.A. system, sometimes I even do porn voices (my boss likes that). I record ad-lib comedies, skits, musicals, and mock-radio broadcasts with my friend who is great at voices as well. I've primarily studied Acting and Vocal Performance (vocalist/singing... not signing). And who doesn't spend hours on Answering Machine Voice Recordings?
I started training for vocal jazz in high school, I continued this through my short stint at a local community college (Kellogg Community College, Battle Creek, Mi... Tony the Tiger says it's Grrrrrrreat). Primarily studying acting and vocal performance. Vocal Jazz and Classical Music tend to train one's voice to be clean, crisp and with lot's of pronunciation.
My friend and I are singer-songwriters and we write and perform (at home) comedy skits and the like. We are looking at breaking into the Voice Acting business and learning more about this wonderful profession.