I'm using a T-Mobile SIM card on 1.1.3 OTB iPhone which I unlocked. Just now I tried swapping in an AT&T SIM which is setup for a Pay-as-You-Go account. This SIM does work in another phone, but on the iPhone, I get a message that I need to add money when I try to place a call. I thought once the iPhone was unlocked, it was unlocked and would work with any SIM. Am I wrong, or is there some other reason this isn't working? Thanks!