What is it the service provider does to a cell phone that keeps it from working on a different service provider?

Welcome to the forums. The carrier 'locks' the device to their network. In order to get it work on another network (if possible since some devices cannot be unlocked to work on other networks), you'd have to get an unlock code from someplace like gsmphonesource.com.

well some device just arent compatible with other services. example TMO and ATT cannot be used on VZW ALLTEL Sprint they use different technologies. GSM vs CDMA VS TDMA

other methods it is a software lock in the phone that doesnt allow the phone to read another carriers SIM.

Thanks for the information. Yes, I uderstand cell phones only work on the services that offer a particular phone listed on their website.

Am I to understand that If I buy a Blackberry on ebay that says they are unlocked, that particular phone will work on any cell phone service? How carfull do I need to be about buy such a phone?

It will not work on ANY other carrier - it must be a 'like' carrier network. For example, if you buy a device that was originally on the AT&T network and it's unlocked, it will work on AT&T & T-Mobile's networks (and Rogers, I think). But you couldn't use the same device on Sprint or Verizon's network since they are different network types (GPRS vs. CDMA)

This may help: BlackBerry Glossary - BlackBerryFAQ

This page is an offshoot of the main Main Page - BlackBerryFAQ website. Can find lots of good info there.

EDIT: I *think* US Celluar is CDMA...but someone else should know for sure.

Cdma

CDMA, same as Verizon and Sprint. BUT, neither Sprint nor Verizon allow devices from other carriers on their network. Nothing you can do to unlock it to use it on a different carrier.

I work for US Cellular and we used to let people bring their equipment over from other carriers, but it just didn't make for a very pleasurable experience. The phones just did not work well because they were programmed for another carrier. The data service was especially bad. I am sure the part about not making money on the phones had a little to do with it.

when RIM makes the BB they are unlocked
they then brand it and lock it to the carriers specifications

sprint chose not to lock their 8830
verizon did

t-mobile & cingular request that their BB are locked to only work on their network.

im sure you know by know all GSM blackberry phones can be unlocked to work on more than 1 carrier

the only one that can not is the 8830