Central Office Calls, Answering

The system allows outside calls to ring and be answered at any combination of system extensions.
Additional Feature Information
Call Handling
Programming for this Feature
Programming Outside Line Type, Access, and Ringing
Programming Line and Loop Keys
Programming Line Transmit and Receive Gain
User Operation
Central Office Calls Answering


The system provides flexible routing of incoming CO calls to meet the exact site requirements.

Answering Priority

When multiple calls ring an extension simultaneously, the system services the ringing calls in the following order:
  1. Ringing Intercom call
  2. Line or loop key (from lowest to highest)


Refer to Overflow for Key Ring Calls in Key Ring for details on the overflow options.

Single Ring Option for Single Line Sets

The option 1403-02: Single Ring for Single Line Set [System: Class of Service: SLTs: SLTs(1403): Single Ring] determines the type of ringing at analog single line extensions. If enabled (Y), analog single line extensions always receive single ringing for outside calls. (Certain analog single line devices connected to the system may require this type of ringing.) If disabled (N), outside call ringing at analog single line extensions follows the ringing pattern of the line. For example, if a line is ringing the system with two ring bursts, the single line extensions will follow the same ringing pattern.

Line Appearance Assignment

When a call rings in, or when the user dials using a line group (9 or 90-98), the system assigns a key to the call as follows:
  1. Line Key
    • If there is a line key assigned to the line, the system chooses it first.
  2. Fixed Loop Key
    • If there is no line key, but there is a Fixed Loop key for the line group of which the line is a member, the system chooses the Fixed Loop key. A Fixed Loop key for another group is ignored.
  3. Switched Loop Key
    • If there are no line keys or Fixed Loop keys for the line, the system chooses a Switched Loop key.

Conditions and Defaults


Default Setting

  • All extensions have full access on all lines.
  • In DSX-40, lines 5-8 require the Expansion Board.
  • Extensions have immediate day/night ring on all lines.
  • Single line sets ring according to 2132-[01-64]: Line Ringing (page 764) programming (just like keysets)
  • Lines 1-8 ring extensions on line keys 1-8.

Release Notes (System)

Line Key Ringing Setup at Super Display Causes Flickering

[3.09] In prior releases, pushing Menu + Ringing + Key Assignments + Line Keys and then pressing the illuminated keys would cause flickering on the telephone. It would also cause error messages to output from the SMDR port. These issues are now corrected.

Reference: A091012003

Other Related Features


  • Call Timer
    • The Call Timer helps a user keep track of the time they spend on the phone.
  • Direct Inward Line
    • A DIL rings an extension directly.
  • Line Keys
    • Line keys simplify answering outside calls.
  • Loop Keys
    • In order to answer outside calls, extensions without line keys should have loop keys.
  • Night Service / Night Ring
    • Night Service determines how outside calls ring at night.
  • Paging
    • Outside call ringing can broadcast over the External Paging speakers.

IntraMail Features

  • None.