I had this. ATT would not exchange the sim so the Apple store actually exchanged the phone. ATT then couldn't activate until they changed the sim number. Phone activated after 48 hours. Last night ATT deactivated new sim and reactivated old sim. They are unable to reverse the process and I must return to the ATT store where they graciously told me I would get a new sim at NO CHARGE TO ME. And then I can activate my phone for the third time.
To avoid my problem - call ATT and make sure that they note on your account that they are telling you it is a bad sim. This allows the store employee to exchange the sim. Can't tell you how to avoid having them automatically put your old sim number back into your account.