Setup a google voice number, and forward your phone to that, at least for a while. You can block unlimited numbers so telemarketers will call for a while and then get disconected message, so that might help (helped us get rid of some daily telemarketers on my cell).
Can you set distinctive rings, such that any unknown number is silent and only known numbers ring? Again google voice can help, as it can be the number you give out for 'home' and when google voice forwards itselfs to your home phone, it can present a caller id of your google voice number, so that your phone will ring.
One varient, you can forward only on no-answer/busy (voicemail forwarding) and then in google voice have the default voicemail be a 'disconnected' recording (google these you can find them around to record to as your message), and for known numbers it will get your personal greeting. You'll want to upload/sync your contacts with google voice to ensure that callers don't get disconnected tone (any new callers or telemarketers would).
This is a variation of this technique to get free unlimited cell calling using friends & family + google voice
http://pogue.blogs.nytimes.com/2011/02/10/the-ethics-of-free-cellphone-calls/