The solution is simple: make sure there is always someone responsible for a spam call.

If I get a spam call, my phone provider should be liable as if they had originated the call. If they got it from another provider, that provider should be liable to them, and so on and so forth. (This would be done by voluntary agreement; only the last mile should be a legal requirement)

All I would have to do is then to report my provider and get the fine. They would then have to chase down whoever connected it to them, and so on and so forth, until we reach Ivan's Shady VoIP Solutions Ltd., headquartered in the Cayman Islands. If they don't pay, no provider would want to peer with them.