The dirty little secret is that the telemarketing companies have their own "DO NOT CALL" lists.  They maintain lists of what they call 'deadbeat' phone numbers, that is, numbers which either never answer, or are always picked up by an answering machine. 

These companies get paid based on the percentage of sales they close.  They won't waste time or money calling a number they know won't result in a sale.  When we moved into our current home, my wife and I used to get telemarketing calls at our new number all the time.  Now, we always let our machine pick up if we don't recognize the number, and pretty soon the calls stopped.  I can't remember the last time we got a sales call for anything.

Regards,
Scott Perry