Whenever you place your SIM in another device and power it on the billing system captures the device IMEI and as soon as the billing system does another sweep you will be getting a text message telling you that you are using a smartphone which requires a data package. If you do not respond they will add the REQUIRED plan without your consent, you will then receive another message telling you that an appropriate plan has been added to your account.
Copied from the Wireless Service Agreement on www.att.com/wireless:
Data Plans: An eligible data plan is required for certain devices, including iPhones and other designated smartphones. Eligible data plans cover data usage in the U.S. and do not cover international data usage and charges. If it is determined that you are using an iPhone or other designated smartphone without an eligible data plan, AT&T reserves the right to add an eligible data plan to your account and bill you the appropriate monthly fee.
