Spam risk caller

That is a notification from your carrier that the number has been reported as a spammer. It’s a new service of AT&T and other carriers are following their lead.

...and I should have added that I upgraded to IOS 13.3.1 last night and I have not received any more Spam Risk calls. I could not find a mention of this as a fixed bug anywhere.

Apple does not make such calls.

All such calls are scams from criminals attempting to steal your personal and financial information. The callers will often use number spoofing to pretend they are calling from a legitimate business. The correct course of action is to hang up on them, repeatedly if needed. 

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

I noticed this last week and wondered why these specific calls with a caller ID assigned by my carrier, AT&T, caused my phone to not respect the settings I made with regard to Silence Unknown Callers. Of all calls I get these are the most likely ones I would want to silence.

They reached my phone! Something in the caller ID info caused this valuable feature (Silence unknown callers) not to work. These calls are the least likely ones that I want my phone to ring on and it sounds like a bug to me.

But why doesn't Silence Unknown Callers not suppress these calls?

Because they never reach your phone. Your carrier intercepts them before your phone sees them.

No. The caller ID information comes from your cellular carrier, not Apple.

Your carrier is telling you that the call ringing on your phone is most likely from a telemarketer.