Unlocked Cell phones????

Will unlocked cell phones only work with T-mobile and Cingular providers???

For the most part those are the biggest gsm companies in the US so yes the will work for t-mo and cing but as far as others, if it's a world phone it should work for any provider meaning that it's a tri-quad band (This is something I would also like cleared up. What's the diff between the 2?)

I have a question and a point of interest. I had a Motorola V600 that I tried to set up as a modem and dial up a local internet provider on my laptop. Everytime it dialed, I would get an error message and could never connect. This phone had a SIM card in it. My new phone,vwhich is a KRZR, does not have a SIM card and will let me use it as a modem which permits me to dial a local internet provider and connect.
Am I able to connect because of no SIM card or does it have to do with the provider? My cell phone provider told it would not work but yet it does. They tried to sell me the card that plugs in to the laptop. I'm confused why mine works when it's not suppose to. Any answers????????

Thanks,
Funster