Sorry for not being clear.
I ask this because it worked with the first gen iPhone.
What you would do is this:
Assuming your current contract with a carrier (other than AT&T) is finished. You would go to Walmart or watever to start a new at&t contract, say with a Blackjack. Have that activated and working. Then going ahead w/ an iPhone purchase and activation, that would effectively renew the at&t contract allowing the Blackjack phone to be freed. Unlock it, eBay it and sell it. TADA!
How else did I pay only $350 for my 8GB first gen iPhone lol.😀
The only reason why this worked, if I have the whole concept down, is that the first iPhone was not subsidized. That meant this could occur. Now ATT will make you pay either an upgrade price or something else that would make it more difficult for you to do this and it would just keep the existing contract you have.
It might work but I'm not sure. I could be completely speaking out of my ass.
I guess I don't know what loophole the OP is referring to so I likewise am lost. I think in order for anyone here to help out we need to know what it is you are trying to accomplish.
For example, do you want to get a BlackJack and the iPhone but only have one contract? If that is correct, you do understand that if it did work you would end up paying $399 (8GB)/$499(16GB) for the iPhone right?
But I am only guessing at what you are trying to ask.