AT&T. But you can "unlock" an iPhone for verizon.
Verizon runs on CDMA. Which means that the software in the phone is connected to Verizon. You can unlock it, but you may not be able to find another company willing to put their software on it for it to run. Sorry I tried to dumb the answer down as much as I can
You possibly can since they both require SIM cards. You might have to unlock your Blackberry though to switch services. I haven't a clue which Blackberry you have, but here is a website that might help. http://iunlockblackberry.com/instructions/
You can unlock your Blackberry 8210 simply enter unlock code using Blackberry keypad. You can get unlock code for Blackberry 8210 using the link below
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Try this website it helps you alot to unlock Blackberry 7100.
Unlock BlackberryThe Blackberry can be unlocked by entering the unlock code on mobile; you can get a code from the service provider.
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