Even between different manufacturers/devices, the VoLTE/VoWiFi support differs. TCL L7 (Yes Edition), in instance, only supports VoLTE from Yes. With other networks (such as Maxis, CelcomDigi), it will have to rely on 2G for calls, and for those with older firmware, these are stuck on 2G (unless you change some settings manually, will reset with each SIM change!).