You can set up a call blocker, that works. I noticed that if one takes off the call blocker and one does not anwser the Sales calls they will call back and mess up the answering machine. I've no idea how they do that but they do it every time. Last year for example I had to buy a new answering machine when they broke my old one.

The routine is like this, you'll get a dead call, no message but a lot of tape is used. Then next time the dead call ends in a squealing sound, then the next the sound as well as the line warning from the TC auto operator and the machine locks up. One then has to dissconnect it and redo the greeting. Final call will lock it up so bad the tape has to be replaced.

I think but cannot prove, the Phone Companies use these tactics as well, especially for added packages like caller Id, Line Blocker etc.