Jaegermeiztr
This issue is probably specific to you. About everyone else isn't having an issue with this (Or called their carrier first or something then got it working)
Do this:
Reboot your phone.
Settings > system > reset > reset mobile
Contact your carrier. Mention what works and what doesn't and that you just got a new phone. Try to avoid saying you installed grapheneOS if you can, they might just latch onto that and refuse to do anything else.
overrides
Settings > SIMs > [your carrier] > carrier settings overrides
It'll say something like:
VoLTE / Vo4G Availability
No override (Default: Available)
"Default: Available" is its current state.
If any are unavailable, confirm with your carrier if that is correct.
Settings > SIMs > [your carrier] > Access Point Names
Confirm the information in here is all correct with your carrier. Confirm your line/SIM is set up correctly on their end.
Godpseed mate