The downlink (tower -> phone) band used outside the US lies partially inside the ISM band in the US. I'm not exactly sure about the legality of it, but you can

The downlink (tower -> phone) band used outside the US lies partially inside the ISM band in the US. I'm not exactly sure about the legality of it, but you can use OpenBTS to transmit only in the ISM band and a quadband telephone will transmit the other direction somewhere rather outside it. You might be able to argue that the phone is out of spec to transmit in that band within the US. What I do know for sure is that I've done this in two places and the FCC Mobile Death Squad hasn't come over for a visit.