"Not allowed" is always a Class of Service issue...
OK but wihch COS ?
WHen the ringing DSS key is pressed, a Directed Call Pickup is performed... So in both the monitored and monitoring sets COS, verify Call Pickup Directed Accept is enabled, and that you have an FAC assigned to Call Pickup - Directed. All limitations and programming required for Directed Call Pickup apply.
Look in the on-board help under System Features - Features C - Call Pickup - Call Pickup-Directed and System Features- Features D to G - Direct Station Select/Busy Lamp Field (DSS/BLF) for conditions and limitations.