the OLAM figures out what phone you are using based upon the IMEI. As the iPhone is carried by AT&T, they know what IMEI numbers belong to iPhones.
Just be sure to swap the SIM card back into the free phone if you ever have to call AT&T (or go into a AT&T store) for service.
see
http://en.wikipedia.org/wiki/imei for full into on what the IMEI number is if you have any questions.