Small fines, for the issuer of the number used to make calls reported by more than N consumers, should do the trick. Small to accommodate false positives. Fine to create an incentive to vet before issuing numbers. Number issuers because they’re less numerous and clearly in the FCC’s jurisdiction.

If one wanted larger fines, N could be lowered but only count complaints with a recording of the call and proof it came from that number (e.g. a telephone bill). Harder to make a complaint, but also harder to turn the mechanism into a home for general grievances.