I seem to be having problems with carrier compatibility - in an EU country - when using GrapheneOS that I do not have when using the stock OS. Everything works fine when I use the stock OS, I can make and receive calls, and although VoLTE does not appear anywhere in the menu (and therefore probably does not work), everything else seems to be fine. The phone seems to be compatible with the carrier, although it's one of the Japanese models.
When I use the same phone and the same SIM with GrapheneOS, I can initially make and receive calls (after inserting my SIM) , but after a while - in a process that I do not understand - calls become impossible, but SMS messages work fine. Call forwarding does not work. It involves some connection to the carrier each time I open that
menu, and that connection fails on GrapheneOS. People calling me get a busy signal, and I can't initiate call, as it's ended immediately.
What could be causing this? Is there a way to solve this without the involvement of the carrier or at all? Could it be carrier-specific?