For a free service you can try the new Google Voice. You can add numbers to a contact list and then set that contact to block. It will play a "this number has been disconnected message". www.google.com/voice
Note the new blog post Blocking incoming or outgoing phone numbers. This new feature will be available as of August 30.
Hi, could someone please tell me how I can keep unavailable numbers from calling me? I need to block numbers that are calling me that do not show their caller id.
If you have a Blackberry or Android phone, you can download an app called SmrtGuard which will let you enter a blocked call list and phone number that you enter will not be able to call you any longer.