Library /sys$common/syshlp/ncphelp.hlb SET, MODULE, X25-SERVER, KNOWN DESTINATIONS, CALL MASK hex-value  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Specifies the mask applied to the incoming call data before it is
 tested.  Hex-value is a string of 2 to 32 hexadecimal digits.  By
 default, no mask is used.