The strategy falls flat when calling a cell phone, but seems to make a lot of sense when targeting landlines. A call from "someone around town" seems worthwhile to pickup.

Add in the fact that people with landlines are more likely to be older people whose priors expect a human to be calling, and the only real question is why don't the spammers catalog and then exclusively target landline prefixes with this method.

It seems like ILECs could start offering a service based on ingress filtering source addrs based on numbers they know haven't been ported out, but Ma Bell is the antithesis of innovation.