Phone spam is when a computer dials your number and when you take the call a recorded message is played. Phone spam is not that widespread yet, but will probably be when flat rate phones (such as VoIP to VoIP calls) become more widespread.
To fight phone spam a human screening facility could be included: The phone owner will record a riddle that the caller will have to solve before being let through. If the caller cannot solve the riddle the call will be diverted to voice mail or dropped entirely.
The riddle may be simple as: "What is 2 + 4?" And when the caller presses '6' he is through the screening.
It is important the the phone owner records his own riddle, as that will make it impossible for computers to get through (as it will require speech recognition and riddle solving).
Other examples of riddles: "Enter 134 backwards." "Spell pat using the phone." "Press star."'
For a more advanced setup the owner can have multiple riddles that will be played at random.
Callers on the white list will not be required to solve a riddle.