I bought an iPhone from a guy I know. I think it's been blocked by the carrier as, when I put a 3 sim card in it works but has no service. But when I put another sim card in it says "Activation required" but when I try, it says "Only compatible sim cards from a supported carrier may be used to activate iPhone. etc etc". I Jailbroke the phone. Is it possible to open it up to other networks?