International phone calls aren't free and unrestricted. It's quite feasible to arrange a regime where such inbound spam calls are economically unfeasible, and legally unfeasible within a regulatory regime e.g. apply large financial penalties to the originating phone company, of which there's a limited number, all are licensed, and can be prohibited from obtaining new licenses.
The non-spammy network operators that interconnect to the spammy network operators make money through that interconnection. If you want them to disconnect their spammy customers, you have to convince them that it's worth it to lose them. This is a manual process and takes time.
This is where infrastructure level legislation comes in. They should be required to provide the full originator information all the way to the people who receive the call, they should block known spammers, they should block other networks that do not do this, and they should be fined heavily if they do not. Otherwise the phone system will become a relic.
