How can I reliably screen or block callers on a number-by-number basis?
I don't use an app.

What I do is have a "Spammer" record in my Contacts. Whenever I receive a telemarketing call, I add that phone number to the Spammer contact.

I give out my Google Voice number exclusively. Within GV I have that contact blocked, so any calls from one of those numbers receives the "ding-ding-ding This number is not in service" message.

In those rare cases where the call is to my actual cell phone number, it quite obviously displays "Spammer" on the screen and I simply slide to ignore the call.