iPhone 3G on AT&T network without a Contract!

I purchased an AT&T locked iPhone which I am using on AT&T's network without a contract because I am only in the US 2-3 months out of the year. My brother has a family plan and I simply borrow a SIM card from him while I am here.

Problem is that I added the Media Life unlimited Internet package to this SIM card account which I was using and got a call from AT&T telling me that I am using an iPhone 3G and that I must have a contract to use it. I explained to them that I only want to use it for 2 months and do not want a contract and am willing to pay whatever amount for those two months and they said no.

Question is, how do they know that I am using an iPhone 3G (IMEI?) and is there any way to get past this as having the phone without Internet access at all is very crippling.

Thanks