This is regarding unlocked phones sold from stores. A major carrier in my country depoloyed a VoLTE network and it worked even on custom ROMs with very minor problems. Now 2 other carriers are following suit but they had to work with phone manufacturers to release an OTA update so VoLTE would be supported by the phones on their network too. Isn't VoLTE a standardized protocol? Why do phones need updates for another VoLTE carrier even though the phone supports VoLTE in another carrier. Isn't it just a matter of changing APNs?