Victim in Moscow complains to Rostelecom. They provide whatever evidence Rostelecom or Russian domestic law requires. Rostelecom complain to Sprint, who now have two choices:

1. Pay the fine for letting their customers send spam calls to Russia.

2. Ignore it, and hope they don't get depeered by Rostelecom.

If they choose the first option, they are faced with two new choices:

1. Stop the buck there, do not contact Joe, eat the loss.

2. Contact Joe and tell him to pay them $100 or get cut off.

This would lead to a self-regulatory process: bad actors would get depeered from the retail phone network, so scammers could only call other scammers.

EDIT: To clarify, Sprint wouldn't have any legal/criminal responsibilities here, everything beyond the last mile is purely voluntary, but they might have a contract with Rostelecom. If Sprint choose the second option, Rostelecom might be able to sue them in the Kansas courts for breach of contract.