content: Probably this, but it will be a technological challenge.  It will depend on how the frequency spectrum is allocated and what agreements towers have between each other.  This is probably a bit easier in areas with shorter required geographical reaches.  If a phone is to be designed for one carrier, the easiest way is to make it frequency specific for their towers. Some carriers have agreements with others to "share" towers, so that every carrier doesn't have to build towers everywhere. If phones were to use specific frequencies only, then there would probably be a lot of people with limited or no service in some areas now, which used to get service.  This might make people even more angry than just not having an unlocked phone.