If VoLTE calls cannot be used, the solution that VoLTE calls fall back to 2G/3G calls (CSFB voice solution) is used to make calls on the 4G networks of certain carriers. The fallback process requires a lot of interactions with the network, thereby taking a long time. (To make VoLTE calls, both the phone and SIM card need to support VoLTE calls. Some models do not support VoLTE calls.)