I hadn't really thought about this before, but your comment made me realize the costs that used to be associated with spam calls in the past.

Monetary or social costs were both asymmetrically in favor of the receiver because a long-distance call was expensive and a local spam call would eventually incur a high social cost (either in the innocent case of teenage prank calls being figured out by parents or the less innocent cases where the police or phone company might need to trace the source).