I have my mobe configured to divert calls from numbers not in my phone book directly to voicemail without ringing - it's easy and I recommend it.
If you ring my home phone you will get a message that says you will need an access code to get through ... "You will shortly be given the access code, however, please note that if you are calling from any organisation other than an employer of someone who lives here, use of the following access code will be considered an offence under the UK Computer Misuse Act (1990). Ok, if you're sure ... the access code is 8362"
Only when you put in the access code does my handset actually ring. The number of unsolicited calls I have had to take since I got this system in place is zero.
An even easier strategy is to record the dah dah dee tone for number unobtainable on your answering machine ... that'll cause most predictive diallers to drop the call.