Technically you can't avoid data plan if you gonna use smartphone on ATT. Whether you provide them IMEI number or not, their system will automatically detect the phone and IEMI number once you swap your sim card into smart phone. You can avoid being detected if you use smartphone purchased from country other than USA. They have all the IMEI numbers of the smartphones sold in USA. That said, you can still use a smartphone and take a chance. I have been using smartphone on ATT since last two years and they have added data plan thrice in between. Whenever they added dataplan, I swapped sim card back to the old feature phone and called them to remove the data. Its working so far.