Comment by gregmac

Money is always the problem. In the carrier world, the party accepting ("terminating") the call gets paid by the party originating it. This is why there are VoIP services that will give you a free inbound-only number and why others only charge for outbound calls.

If you're a carrier, it pays to terminate all calls -- spam or not -- by delivering them to your actual customer. You get paid by the originating carrier, and in a lot of cases you also get to charge your customer per-minute fees (or use up their prepaid minutes).