Putting my Sim card from an iPhone into a different iPhone?

I'm an AT&T Carrier. I already have an iPhone 4 data plan that I'm paying on. Recently my phone has stopped working and until I can get a new one shipped my friend has offered for me to put my sim card into her old iPhone 4. Will AT&T charge me if I do this?

They shouldn't. My friend switched his SIM card one time. He was with a different carrier though. You always have to be careful when doing this. You should call them or check their website.

No. Your plan works off the SIM card.

As long as your friend's phone isn't locked to another carrier, you should be able to just put in your SIM and go. There's no reason why AT&T would charge for this; they may not even notice. The subscription depends on the SIM, not the phone, although phones are often locked to accept only SIMs from a specific carrier, if they were bought at low cost associated with a subsidized subscription.

Why would they?

The more important question is if the other iPhone would be able to accept the SIM. If it is also from AT&T then it would.

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