If there is no number displayed, then you cannot block it. I get plenty of the calls as well and really even contacting my phone network provider cannot stop them.
Phone app, three dots then settings. Then you will see top of the page Block unknown and private numbers. If you set the toggle to do this then all calls in these two categories will not be picked up.
Definitely works on the s21 Fe using the method above, I blocked my home phone by accident then wondered why it was just giving an engaged tone (doh!) Our telephone number is x-directory/withheld (not that anyone calls it nowadays anyway...) Kinda freaks me out when it starts ringing
No doubt, I had it enabled on my s8+ I just never enabled autoblock, too be fair I've had to disable the autoblock on my 21 as it blocks messages from my gp, the smart caller id is a nice feature though more so when it has a successful lookup
