When rejecting calls, all subsequent calls go straight to voicemail

when my dad rejects a call on his iphone, all subsequent calls go straight to voicemail and phone doesnt ring when I call him. Does anyone else have this problem? this has persisted for a very long time. and I tried to help him solved it over a year or so ago. I even went to the lengths of contacting AT&T and asking them to reset something on their end (which a user suggested on the internet) and nothing solved it. Issue still persisting. He has iPhone 15. We've done a ton of troubleshooting with no resolution. he is not in airplane mode. he doesnt use focus. the phone isnt muted (because if it was, i would hear it ring when i call him, but it just goes to voicemail). phone is up to date. issue persists.

[Re-Titled by Moderator]