After the newer versions of iPhone was launched people were hesitant to switch to it as AT&T no longer offered the unlimited data plan for $30 per month. All you could do was to go for $15 per month plan which lets you use 200MBs or the $25 Pro Data plan that lets you use 2GB of data per month.
If you already had $30 data plan then you are still good as you can continue with that. If not than you can still follow the instructions below and get $30 unlimited data plan (This method can also be used to get rid of data plan):
1) Find an ordinary phone or buy it off craigslist or e-bay for few bucks. Note down its IMEI number.
2) In the general settings of your iPhone see the ICCID which is the sim information.
3) Call AT&T and talk to a representative. (You might dial 611 and then press 1, 0, 0 to talk to a rep without listening through the recorded messages)
4) Ask the representative to change your data plan to $30 unlimited plan as you are no longer going to use your iPhone.
4) Ask them to take the data plan off as you are moving to a non-smart phone.
5) If they say that you can not use your iPhone sim on any other phone tell them that you have a sim adapter that works on your non smart phone.
6) They should confirm the changes to you now and you don't have to do anything. You might receive some texts from AT&T but in a few hours you will be good with unlimited data plan for $30 a month or no data plan.
For more applications and similar hacks visit Free Calls Hub.