01- put your sim back in your old phone, call att & deny that you have an iphone, and have them not change your plan. Put the sim card back in the iphone after the phonecall. Rinse and repeat. And hope they give up before you do.
02- call att and tell them you never purchased an iphone with them - and you do not authorize them changing your plan.
03- Get out of your contract and go with somebody other than ATT and keep your iphone:
tmobile phone plans
plans/Cell-Phone-Plans.aspx? catgroup=Individual&WT.z_shop_ plansLP=individual
04- change the IMEI of the phone so that ATT cannot tell it is an iphone (may cost upto $50)
>> Can only mask IMEI on 2G (4.6 BL iPhone)?
>> iPhone | How to Change Your iPhone IMEI with ZiPhone (Windows)
I tried to use my sim with a Medianet plan on an iphone too and I faced the same problem of not being able to
connect to the internet. I did some research and realised that the phone was missing AT&T’s data settings. Hence I went here and installed the proper settings. Although tethering did not work on my phone (probably because I was on iPhone 3.1), the data connection went active.
FYI - Boost Mobile's phones and network are iDEN, not GSM. The iPhone is a GSM phone.
Even the iPhone GOPHONE is unable to access the web.
>> Unlock CNS (Carrier Not Supported) iDEN i465 Clutch Boost by JIC Technology
>> Give Your Old iPhone New Life With Prepaid Data and Minutes
>> Turn an Older iPhone into a Prepaid Voice and Data Unit