bryan
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
https://github.com/GrapheneOS/os-issue-tracker/issues/956
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.