Joe has five late model Motorola Droid phones with Verizon, but he wants to change carriers. Leo says that Motorola Droids only work on Verizon. If he unlocks them, he could take them over to Sprint, and maybe AT&T or T-Mobile if the frequencies are supported. He'll have to get the SIM slot unlocked. He should tell them he's going overseas and needs it unlocked to get a local SIM. PadreSJ says a DROID MAXX can support an AT&T SIM Card, so if he has a friend who has one, he could try it.
Samuel is coming to the States next year and he needs a recommendation for a smartphone and carrier. Leo says that T-Mobile may be his best bet because it's GSM. AT&T is another option, but Leo prefers TMobile. He'll need an unlocked phone, then it's just a matter of buying a SIM.