How do you unlock your cell phone so you can change the Sim card to another cell.?

My sister moved to t mobile but her old phone was cingular and when she asked them to unlock her phone it became a problem. she should be able to use her old phone and move the sim card. I know someone knows how to do it?How do you unlock your cell phone so you can change the Sim card to another cell.?
Just open the back up, remove the battery pack, and take out the card. Then put it in the other phone.How do you unlock your cell phone so you can change the Sim card to another cell.?
generally you would pop out the batter and look for a small plastic chip about half the size of a postage stamp held in place by a metal clip just switch these and the service and contact list will go with the chip in the exchange.

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