Developer of RoboStop here. If anybody here in the comments section happen to use an Android phone and are plagued by telemarketing calls from Indian dev shops offering to build you a website, check out my app! http://robostop.org. It's pretty simple, just hangs up on anyone not listed in your contacts. Very effective. Just make sure to turn it off when you order an Uber.
P.S. it's not on the Play Store anymore because I got booted off along with all of the other apps which access users CALL_LOG(s).
I would not want that for the same reason. It's not great but I deal with them by 1) generally keeping my phone on silent and 2) only answering calls from people in my contact list. If they really need to reach me they will leave a voice mail, if not then it most likely wasn't important.
I'm not sure a cop would leave a message in the situation you mentioned above but otherwise it works OK. Some of the robo callers end up leaving a message but it's almost always 2 seconds or less so I don't bother listening, anything that short just gets deleted.
P.S. it's not on the Play Store anymore because I got booted off along with all of the other apps which access users CALL_LOG(s).