Calls do not work
If calls do not work, it may be for any of these reasons:
No service or Emergency calls only
Refer to No service or Emergency calls only.
The phone does not support VoLTE for your carrier
The ability to make and receive calls on LTE without switching to 3G/2G for the duration of the call is not base functionality and your phone must support it for your carrier specifically. For example, phones intended for use in other countries might not support it for your carrier. This means 3G/2G must be enabled in the phone's settings and 3G/2G coverage must be available in order to make and receive calls, i.e. not "LTE Only".
The phone does not support VoNR for your carrier
The ability to make and receive calls on 5G without using LTE is not base functionality. If the phone is on "NR Only", correct the misconfiguration.
Misconfigured supplementary services
Ensure call forwarding, call barring, CLIR, etc are not misconfigured.
Misconfigured user equipment mode of operation
If you changed it, correct this misconfiguration.