My first exposure to voice-over work came when I was working in Japan. I worked in content development for English language education software, and did all of the male American voicing for a 12-CD set. Unfortunately, I have no access to those audio files.
I've not done voice-over work since then, but in my job where I run a ton of conference calls, I generally get told every week that I have a great voice and should seek voice over work.
My training consisted primarily of five years of teaching classrooms full of Japanese students how to speak English. I had to maintain clarity and diction, while at the same time projecting and maintaining volume.
I work in marketing and feel that this gives me an edge in understanding what the client would like me to deliver