Namely, a new set of APIs that will handle SMS and Call blocking was found hidden in the code by developer Walking Cat. The strings of code imply that the new feature will allow Windows Phone 8 users running the latest iteration of Microsoft's mobile OS to block calls and texts from specific or private numbers or simply block numbers outside your phonebook altogether.
If you're running one of the few devices that run GDR2 already, like the HTC 8X or the Nokia Lumia 1020, you're probably wondering how come you haven't come across this feature yet. Simple – while technically supported, Windows Phone 8 hasn't got an interface for the feature yet. Such vacuum is rarely left unoccupied for long, though, so it probably will be only a little while until we see the first set of third-party apps taking advantage of the new feature pop up.
source: @h0x0d via WMPowerUser