--valid numbers that have been allocated to a phone company but haven't been assigned to a subscriber: in a carrier's reserve
First let me address your quote. We broke up Ma Bell and created CLECs remember? You can get a phone number from multiple carriers. Or to make it easy, Sprint has no idea what numbers I have registered from T-Mobile.
Now on to what really matters. When your attacker (lets call them what they really are) are coming from a foreign vpn using a legitimate US VOIP service what do you actually do? The VoIP service typically does terminate the account and moves on with their day. Now you have an entire call center in a country that doesn't necessarily follow US laws that (a) purchases VPNs or dedicated servers/colos from US based companies, (b) opens multiple accounts with multiple VoIP and Cell phone services and starts the process all over again.
What you need to do is attack the company at the end who is charging credit cards. Why do I need to spend several weeks to capture the famous robo-dialer "Rebecca from card services" and find out one of it's users is a debt consolidation company in Florida. I get all the details and pass it on to relevant authorities, including alerting my credit card company that this is a fraudlent service, who do NOTHING. The Florida company is still open and running today, even though they claimed they were using a lead-generation service. The laws are weak. Until you punish the companies benefiting (Credit Card merchants, Lead-generation Users, etc) you won't see a change. And that won't happen under this presidency so lets keep on dreaming.