But doesn't AP setting mainly affect hotspot functionality?
My understanding is it affects: MMS, cell internet, hotspot.
Voice and SMS usually aren't affected by the APN.
https://apn-settings.com/apn-troubleshooting/
Many issues here are rather rare and should work or is a 1 in 10,000 mistake by their carrier.
So many people have a carrier issue on a brand new phone or sim and resetting carrier settings fixes it.