Hi there I’ve just been having the same problem this is what worked for me
Prepare a SIM card that supports VoLTE functions
AT+QCFG=“ims”,1 //enable IMS (VoLTE)
AT+QMBNCFG=“autosel”,1 //enable auto selection of MBN file based on SIM IMSI
AT+CFUN=1,1 //reboot to take effect
Test VoLTE
AT+COPS? //check if module is in LTE technology (7=VoLTE), else VoLTE can’t be tested, as SIM does not attach in LTE
ATDnnnnnn; //voice call to number nnnnn, can be local or international number
AT+COPS? //check if module is still in LTE technology (7=VoLTE, test passed) or it re-registers to 2G/3G (0/2=no VoLTE, test failed) during the call
ATH //hand up the call
AT+COPS? //check if module returned in LTE technology (7=VoLTE), if previously was 2G/3G duirng the call