There's not? Is it required to answer the telephone in any way? Who can they harass if nobody answers their calls? You won't hear anything from anyone you don't want to if you completely ignore unknown numbers. They've got to have some sort of answer to deliver threatening messages, take that away and hear nothing but a ringing phone. Maybe consider a call blocker or a blocking app for a cell. Not answering at all is free of charge, though, and gets on callers' nerves if they're drawing blanks on the numbers they call.