Set up your phone such that when you don't answer a call from a number you don't recognise, it will go to voicemail. Set up your voicemail answering message to

Set up your phone such that when you don't answer a call from a number you don't recognise, it will go to voicemail. Set up your voicemail answering message to contain only two things: the tones that signify "This Phone Number Not In Use" (see http://www.k3pgp.org/telezap.htm http://www.k3pgp.org/telezap.htm) and a terse voice message, such as "Leave a Message", which imparts no usable information to the caller. Now when the call goes to voicemail, two things happen: First, any robocallers will hang up when the hear the 'Not in Use' tones. Second, any humans will hear the 'Leave a Message' and they now have two options which are to hang up, or to leave a message asking you to call back. You merely have to scan the voicemail messages and call back only those ones you choose to answer. The other 80% or more have been discarded 'automatically' before you get to hear them.