The Busy Lamp Field (BLF) is a cloud PBX feature that monitors statuses of individual phone lines (idle, busy, etc.) within the same cloud PBX environment and displays them in real-time on the attendant phone console (IP phone with BLF). Thus, a user of an IP phone with BLF can see which of their coworkers’ phone lines are idle/busy at that moment, to decide, for example, who to forward an incoming call to. PortaSwitch supports the BLF feature, which extends the capability of PortaSIP to work with popular IP business phone models (e.g., Polycom).

IP phones that can use BLF have a “field of lamps,” wherein each lamp reflects the status of a phone line. The behavior and color of the lamps differs depending on the phone model, but the most popular and intuitively perceived ones are the following: ‘off’ for non-subscribed, ‘green’ for idle, ‘blinking’ for ringing, and ‘red’ for busy. In some models, the lamps are combined with buttons that allow a user to perform further actions (attended/unattended call forwarding, conferencing, etc.).

Requirements and configuration

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The BLF feature is implemented through the SIP protocol and uses SUBSCRIBE and NOTIFY requests. Since BLF feature support is embedded in PortaSIP, no additional configuration is required.

To indicate the status of particular phone lines, the IP phone with BLF must be subscribed to notifications regarding those phone lines. The exact procedure depends upon the phone model; refer to the User Guide for the respective phone.

For security purposes, only accounts within the same cloud PBX environment can monitor each other’s status.

Note how the BLF feature works for an account with Override identity functionality enabled. When an account user takes part in a call, the change in phone line status is indicated for both their account and accounts/aliases defined as the Identity and Display number. Let’s say the Override identity feature is enabled for 123456 to be an alias for account 555001. The identity value is defined as 555001. So, when 123456 makes a call, IP phones subscribed to both phone lines (123456 and 555001) receive notifications about their status change from PortaSIP. If 1212567690 is defined as the Display number, its status also changes when 123456 makes a call.

This allows monitoring calls of the main account (phone line) and all aliases associated with it. For more information about the Override identity feature refer to the SIP identity chapter.

If the SIP contact feature is enabled for an account (phone line), an IP phone with BLF subscribed to this phone line doesn’t receive notifications about its status changes when this account receives incoming calls.

The BLF has been tested and provides full functionality with IP phone models listed in APPENDIX D. SIP devices with supported BLF.

How it works

Step 1. Preparation: 

  • The IP phone with BLF sends a SUBSCRIBE request to the dispatching node (1) which forwards it to a currently available subscription manager.
  • The subscription manager authorizes it in PortaBilling (2) and records the subscription in the database (3).
  • Based on the results, the subscription manager sends a response (a 200 OK SIP response) via the dispatching node back to the IP phone with BLF (4).
  • The subscription expires over time and can be periodically renewed by repeating the above procedure.


Step 2. Call flow:

When the IP phone with BLF is subscribed to the phone line(s) any change in the phone line status (to idle, ringing, or busy) is reflected on the Busy lamp field of the IP phone. Thus, if phone 1 calls phone 3, the call controller establishes the call between the phones (4), checks for active subscriptions in the database (5), and notifies the subscription manager(s) who handle(s) the presence of these phones (6). The subscription manager updates the status of phones 1 and 3 and sends the NOTIFY request to the IP phone with BLF (7). The NOTIFY request includes XML with dialog-info about the current status of the phone line being monitored. The IP phone with BLF acknowledges the NOTIFY request by sending a 200 OK SIP response (8). BLF call flow

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