That's what I'm getting at. If robo-callers are just calling automatically, any combination of numbers from 111 111 1111 to 999 999 9999 can be called. So eventually any land or cell phone number will be called. If the phone rings, computer knows it's a working number. If the phone is answered (human or machine), computer knows there's likely a human around. Validated numbers are then sold to other telemarketers (or scammers).
Note - the "Caller ID" number that shows on your phone isn't necessarily the number that called. I've received many that show 123 456 7890 or 999 999 9999, etc. You can't call them back, and if you report the number somewhere, it's not a working number.
Blocking a number isn't always effective, as the Caller ID number isn't the number that actually called you.
Hmmm - never got a junk call on my tin can and string ...
T.