Basic 3G Calling & LTE works even Google does not support your carrier VOLTE functionality. For SKT/KT, 3G Calling & LTE Network works fine. But for LGT, You should have VOLTE enabled to use Calling/SMS. So If you're using LGT, You cannot use GOS without ROOT. There's a hack for enabling VOLTE for unsupported carriers but this hack does not guarantee VOLTE functionality and could make security concern.
Solution fir this issue is planned but no ETA
If you have Java knowledge, You can contribute codes.
Carrierconfig(carrier_volte_available_bool) is closely related to the solution.
Also, To use VOLTE flawlessly on Pixel 6 on unsuppprted carriers, Google & Carriers should solve it. ROM developers cannot solve fundamental problem.