When I used to get a lot of spam calls (because I had to answer my phone for everything), I used to answer numbers I didn't recognize as "Orleans County 911, what is your emergency?" Most spammers got the hint and either hung up immediately or stammered out an apology and hung up.

One person insisted on trying to sell a car warranty so I ramped it up a bit. I'm only a little sorry that the person was apologetically crying about "interfering with emergency services" by the end of the call.