1. u dont have to unlock an iphone to use another AT&T sim card.
2. just make sure ur sim card has a fiery ball with 3G symbol on it.
3. dont let ATT know that u gonna use iPhone.
4. call csr and tell them u want to put datablock soc code on ur number. datablock will block internet.
5. they will scare u by saying that iphone automatically updates itself so u gonna accrue more $$ charges if u dont get dataplan. that is totally false. u can disable automatic update in ur iphone to avoid that.,
whatever dharane kancha said is  true.. thts how an att rep will "educate" u if u sonsult with them.
6. they will know u r using iphone only if u give them ur iphone IMEI number. with iphone IMEI no, AT&T's software mandates dataplan so use ur Raz IMEI, or in other words, as i said earlier, dont "update" imei in their system.
7. AT&T follows the moto " Customer is always right" so.. even if their system detects iphone and sends u sms.... u can disregard it.. or actually call them and tell them tht they are harrassign u by sending iphone alert messages in ur moto razor phone. to be on the safe side, put ur sim in moto raz when u call them.