You could try a more complicated approach: sell consumers a device which records their calls locally, and sign them up to the do-not-call list. The device would have a spam button; when pressed it would send the caller id + recording of call to your company's servers. The company would use this to help the customer threaten or sue telemarketers, splitting settlement/lawsuit revenue.

This would be illegal in many states, but you could restrict it to states with one-person consent to record conversations.