This is a read-create setting that defines the action applied to the 'D' signaling bit of the incoming ABCD pattern, completing the set of four independently configurable bit actions alongside A, B, and C. An admin configures this when the D-bit's role in the CAS signaling scheme, often used for ringing or additional supervisory states, needs to be inverted or otherwise remapped to match the far-end equipment. For example, integrating with older PBX hardware using a nonstandard D-bit convention would require setting this action to align the two systems.