CO Lines
From Bizfon Wiki
Purpose of CO Lines
In TDM-based PBX, there were a number of physical lines connecting the PBX with the public telephone network. These lines were called "CO-lines".
It is interesting to see what calls are active on the CO-lines. Over the centuries that PBX have been used, office users got used to CO-lines and they do expect at least the same behavior from a modern SIP-based PBX.
In SIP, there are no more physical cables used to connect the PBX with the outside world. However, it is still interesting to see which calls are active between the PBX and the outside world. Therefore, the pbx emulates the behavior of the TDM-based PBX.
CO-lines are associated with trunks. Each trunk may have several CO-lines. Because users can subscribe to the state of the CO-lines, their name must be unique in the domain like for all other accounts.
For example, you can set up four CO-lines on Trunk1 with the name "co1 co2 co3 co4" and more CO-lines on Trunk2 with different names like "co5 co6 co7 co8" (the list of CO-lines must be separated by space). The PBX will reject names for the CO line that are already used by accounts or other CO-lines in the same domain. The CO-lines are listed in the account list, because they share the same namespace.
Limiting Inbound and Outbound Traffic
Having a limited number of 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 of the line for each call. When all lines are in use, the PBX will reject further calls that would use the CO-line.
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.
