Don't answer the calls, let it go to voicemail (trying to unsubscribe is self defeating). Use the telco SIT tone on your outgoing voicemail. The tone tells automated dialers that the number isn't valid. Google search SIT tone wav. Play the tone off your computer as the first thing on your voicemail recording, wait 2 seconds, then speak the rest of your outgoing message. Mostly works for automated dialers, but some human dialed marketers hear the first part and just hang up.
If this board allows it, the link to the SIT tone wav file is here:
http://www.telephonetribute.com/audio/sit_disconnected.wav
I went a bit further and put this together for my voicemail message:
http://www.jon.info/outgoing.wav
There's also inexpensive devices out there you can connect to your land line phone that play the tone whenever you pickup.
Jon.
[email protected]