CalcProgrammer1, Someone over on GitLab did some really impressive work into reverse engineering the Qualcomm IMS bringup for VoLTE service on Linux phones. I tested his proof of concept IMS data service tool and was successfully able to place and receive calls on my #postmarketOS #oneplus 6T from LTE/4G mode! Audio worked in both directions! IMPRESSIVE!
Check out Richard Acayan's project here:
https://gitlab.com/flamingradian/81voltd
VoLTE discussion here: