Even I have been facing the same, switching automatically to 4G for voice call is not a smooth process and leads to inability to call or receive calls. It's a trade-off in exchange for unlimited 5G data and will remain that way till Jio manages to deploy 700 MHz pan India and achieve blanket coverage like 4G, then they will deploy VoNR for voice calling using 5G network without needing to fall back to VoLTE.