Comdial DID CO Line Board 4 Port DXPCO-DD4 Features: – The DID line board (DXPCO-DD4 and DD8) allows incoming CO callse to reach internal DXP intercom extensions by direct dialing. No attendant assistance is necessary. Since DID lines are incoming only, their direct appearance is limited to attendant stations where status indication may be useful. DID operation requires a group of published directory numbers (400 maximum) provided by the telephone company central office (CO). These directory numbers are incoming only and the DXP translates them to the appropriate personal or group intercom number for ringing.