Yes you can if they use the same bandwidth...but why would you want to do this... HMMMMM?

There are a couple issues to be addressed here. What phone company was your phone "locked" to? If your phone was originally a T-mobile phone, you have to get the SIM unlock code (aka subsidy unlock code) from T-mobile, and the same applies if it was originally a Cingular phone. The phone will probably work with Cingular once it's unlocked as long as it's a GSM phone and works on 850/1900 MHZ signals.