cabbie

I didn't know about this until i did a program on it, but with some (maybe all, but since i don't know, i'll stick with some) companies kick you off their service when you use service through a tower not at the location you registered with.

I don't know much about it so i was wondering:

1) what is the percentage that is allowed off-network
2) is it only when it's through another's company's tower, or just any tower (even your company's) that is not going through where you're registered
3) is this true for all companies or just some?