Phones and Sim cards

Ok so my best friend has tmobile and i have at&t or cingular and she wants to give me her blackberry pearl for tmobile. I've heard that you can trae sim cards with phones on different plans but is that true. Can i take my cingular sim card and put it in her blackberry and expect it to run just fine? I need help

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The pearl needs to be unlocked before it will work. You can swap sims if the phone is branded to the carrier of the sim, or is sim unlocked. In this case, since you have an ATT sim, and the phone is most likely locked to T-mobile it will not work as is. Have your friend contact the carrier, they may unlock it for her. Or go to a commercial unlocking company and they will do it for a price.