Google Voice enables pretty intelligent call routing. One of the excellent features enables you to add contacts to your address book right from the web interface online… this is the same contacts list available in your gmail account which keeps things really streamlined.
In contacts, you can have "Groups", such as Friends, Family, Work, Badminton-Chums… whatever.
If you get a telemarketer’s call through your GoogleVoice line, you are halfway to getting those call’s ignored completely.
Simply create a group called something like "telemarketers" or, my favorite, "Spam" and then add all would-be naggetty-nagging call-center callers to that group.
In Google Voice, go to the "Voice Settings" menu, click the Groups tab, and then next to your "Spam" group, click "Edit" (If your group doesn’t show up in this list (mine didn’t), click "Manage Groups at the bottom. You will go to the contacts management page. Find your Group on the left panel, and click "Edit Google Voice Settings" on the right).
Then, uncheck all your forwarding phones and set the voicemail message to "System Standard" or create a custom message like "Hello. Please add this number to your do-not-call list and do not call any more."
They could, still, leave a voicemail, but hey, you’ve got Google Voice, so you can see what they are saying to you in text, email, online or in the app. Just delete it if you don’t like it. No-problem.