Default [Q] blocking unknown calls...

on my old S4 i could block calls that show up as "unknown". can't seem to do this on my M8, only block numbers. is there anyway to do this with the stock M8 dialler, or do i need a third party app?
Phone: HTC One M8
Rom: ViperOne M8 2.4.0
Kernel: Stock + S-OFF
Network: O2 UK