I have the iPhone XR and despite changing the settings on my phone to allow all incoming calls, my phone just sends the unknown numbers straight to voicemail. I can not figure out how to fix this.
iPhone SE, iOS 12
In the UK with an iPhone 12 mini with French mobile account (Free Mobile) on eSIM. Could call out but all incoming diverted to voice mail.
It turned out to be a setting on Free Mobile account sending all FOREIGN calls to voice mail (only calls from UK numbers were tried (seen as "foreign" by French system). Presumably, calls from French numbers would have got through.
Turning off the phone and restarting solved the problem in seconds.
Have you checked: Settings - Phone - Silence Unknown Callers = ???
Unknown incoming calls go straight to voicemail
