The FCC got the flagging backwards. If a number is unverified, this unverified number should be appended with a flag...not the verified numbers.
For example, if an unverified number is preceded with a , you could block the number by its flag. Of course, you could only do this as a wildcard... 123-456-7890, 000-000-0000, etc. All numbers preceded with the get blocked.
If Cox offered this option, Cox would (naturally) charge you for the service. If you had a local call-blocking feature on your telephone or as a standalone device, which supports wildcarding, it'd be a cheaper option.
As with telemarketing crackdowns, the Do Not Call Registry, Nomo and this program (STIR/SHAKEN)...another gov't flop.