AAT&T told m that the sim card is connected to the phone and that you can not use it with another phone without going through an activation process. I had asked them when I was buying my son his first phone, I was hoping to add a new line with a really nice phone and give him my current phone and use the one that I bought for my phone. They told me that my phone is locked to my number and that his would be locked to the new phone. If I wanted to do that I would need to pay separate activation fees for each phone in order for them to work with the different sim card then they were assigned to. Now with all that said, I have put my sim card into his phone and had all my contacts show up and I can use it as a phone but the data and stuff that my plan has does not register on the phone. My guess is you can do it but AT&T will want you to pay an activation fee for that new phone to be recognized with your plan.