it's been done. i've heard of people using sprint phones on verizon. not recommended though, for the following reasons. (By the way you can read about these on any Verizon or Sprint message board. A lot of people want a PDA that's offered by Sprint on Verizon, or the Virgin slider phone on Sprint's network on Verizon, and so on)
1. Internet will not work (Sprint uses "Vision", Verizon uses "Get it Now". both have different authentication processes. but you knew that, didn't you dean..hahah)
2. Voicemail tends to have issues. I'm not sure why. I've thought about that for a while. But people who have done it report that their voicemail never quite works.
So its been done, huh? interesting.
yeah #'s is weird, voicemail has nothing to do with the phone, its all automated at the carrier's location. They just mad you using a phone not approved for their network and getting back at you.