CO Lines

From Pbxnsip Wiki

Purpose of CO-Lines

Public telephone lines connected to traditional TDM-based PBX systems are commonly referred to as "CO-lines". Over the years that PBX systems have been used, office workers came to depend on CO-lines to monitor activity.

While pbxnsip runs calls through SIP trunks over computer networks instead of connecting to public telephone lines (at least directly), it is still useful to see which calls are active between the PBX and outside world. Therefore, we have added CO-line emulation to pbxnsip.

CO-lines are associated with trunks. Each trunk may have several. Because users can subscribe to the state of CO-lines, each CO-line name must be unique within a domain. For example, four CO-lines might be setup on Trunk1 with the names "co1 co2 co3 co4" while Trunk2 might contain "co5 co6 co7 co8" (the list must be delimited by spaces). pbxnsip will reject names for a CO line that are already in use by accounts or other CO-lines in the same domain. The CO-lines that you establish can be monitored from the Domain's Account list. If a CO-line is in use, it will display the extension and outside phone number to which it is connected.

Limiting inbound and outbound traffic

CO-lines can be used to limit the number of calls that can be assigned to a trunk. When the CO-line setting is used, the PBX will reserve one line for each call. When all lines are in use, the PBX will reject further calls that would use the CO-line. For example, if you have a SIP trunk that can only handle three simultaneous calls, you should assign three CO-lines (eg "co1 co2 co3"). If a fourth simultaneous call attempt is made, pbxnsip will play a fast busy signal. If you have more than one trunk, you can setup a Failover Behavior (dropdown field on the Trunk) so that pbxnsip will then try to use a different trunk for the call once the first trunk has all CO-lines in use.

Sometimes, it makes sense to reserve lines exclusively for inbound or outbound traffic. If you put a “:i” behind the line name, the PBX will use that line only for inbound traffic (e.g. “line1:i”); if you put a “:o” behind the line name, the PBX will use that line only for outbound traffic. If there is no attribute set after the colon, the line will be available for inbound or outbound traffic.

Monitoring CO-Lines

In most small offices, transfers are being done by parking and picking up calls from lines. In order to be able to do this, it is necessary to display on the phones which call is on which CO-line.

From a PBX point of view, the status of a CO-line is similar to the status of an extension: It may be idle, connected, ringing, on hold or there might be a call being parked. Therefore, the mechanisms to see the status of a CO line are similar to the mechanisms to see the status of an extension. This implies that the name of the CO-line must not clash with the name of an extension or any other account on the system.

In order to see the CO-line status, the user agent needs to subscribe to the status of the CO-line. See the description of the phones on how to do this.