It looks like some new features for Windows Phone 8 have been discovered by developer Walking Cat, these new features will be released in the Windows Phone 8 GDR2 update.
Two new features for blocking calls and SMS messages have been discovered by comparing the Windows Phone 8 and Windows Phone 8 GDR2 emulator code.
It would appear that the new GDR2 update features new APIs for blocking both text messages and calls, and it will come with the ability to block specific phone numbers.
Source WMPoweruser