No return #? Yeah that sounds pretty irritating. Are you on the do not call list?
If you have caller ID (if not blocked) note their phone number. If caller ID is blocked, use *69 to call back last received call and note what number shows up. Then publicize that number widely (perhaps on FR ;) Someone out there might have a Robo-caller of their own that coincidently would repeatedly call that number. Or just a lot of average “Joe the Plumbmers” might repeatedly call that number. They can’t call out if their line is busy with incoming calls.
Note, don’t attempt to do this from any Repubican campaign phones.