I have some experience with this. When I purchased a new Boost phone, what I did was not activate it with their included sim and buy the unlock service before I activate anything. Then it is possible to use with AT&T and T-Mo and international GSM. However, there is still an "Unpaid bills" flag on the phone, this make it not possible to be activated on Sprint until the flag is cleared, probably by using the phone with Boost first, or buy a service to clear the unpaid bills flag from that IMEI, which is different from sim unlock. I don't know about though Verizon.