iPhone 12 Pro

Half ring then to goes to voicemail. - Apple Community

This relevant thread maybe helpful and comprehensive for you to solve the issue. Please check :)

SLChaudoin wrote:

My iPhone 12 Pro either rings once or not at all and then goes to voicemail. The calls show up as missed in my recents. I’ve done all the suggestions I could find on the internet. Help!

And what exactly is "all the suggestion I could find on the internet"? So that way someone doesn't respond with something only to have you say I tried that and it didn't work.