Often it's the slime trying to sell you credit, although I seem to have scared enough of them off recently. (In the guise of Mr G. Yusuf)
Why on earth should Volvo Cape Town not want me to recognise their number when they call me?
If enough people simply bounced all "Unidentified Callers", would they get the message? Probably not, the corporates are not known for their ability to listen.
In future, all "Unidentified Callers" will be answered with
"This phone does take not accept calls from unidentified callers"