I've recently changed our answering message warning if this is a telemarketer or solicited of any kind, we are register with DoNotCall.gov and they will be reported immediately. None of these callers ever leave messages or ever call a second time. And I do report them.
The simple solution is to turn off your answering machine and when caller I D shows an unrecognized number, just let it ring. I usually do an internet search to verify that the call was properly ignored. If not, it's easy to call back the legitimate caller. If these clowns don't get an answer, they eventually give up.
Or do as my ex does and tell the caller — in the rare case when there is a live person calling — and tell them it's a real coincidence that JUST LAST WEEK you just did whatever it is they are promoting. Ha!