Hi everyone,sorry I searched sim cards and wasn't finding exactly my question so here goes: I am buying my friend's 3Gs iphone, and I have already a 3G. Can I simply swap my sim card into her old phone, and continue using my current plan and number through AT&T, or do I have to call AT&T to do any activating of this new phone?(her old one) I'm hoping I don't have to call them at all (what a hassle). Can anyone let me know of their experiences doing this and if that is all there is to it or is there more? Thank you in advance!