Many countries ban phone spam or have a centralised opt out list. A good side of the regulated telecom world.
edit: replying to comment below, many countries have working opt out lists.
edit: replying to comment below, many countries have working opt out lists.
My phone should be saying to itself "Hold on a second, am I about to turn on the buzzer and display the text "Unknown caller" to the users? /dev/null it."
Maybe people don't know how to select this feature when they make calls? At least in GSM based networks you can select it on a per call basis when dialing (#31# prefix).
Eg. calling in anonymous tips to your local newspaper, communicating with an annoying company you don't want calling back, etc.
Of course, sometimes I get calls from actual humans for polls or advertising a product, but it's much less of a bother since it happens less often and you can reject them.