When it comes to smartphones like the iPhone u going to need to have a data plan on the phone in order to just use it for calls and so on and so fourth.
When u get the iPhone and put your sim card in it, after a few days (once the att system updates that your imei on file) You account will be flagged, and u will have a restriction on your account, and be talking to the equiptment gaming team, trust me, u dont want to mess with them, they have too much power, corporate cant even touch them.
The only way u can use a iPhone on Att without a data plan is if u use it on prepaid.