What Is Busy Lamp Field (BLF)? How Does It Work?

One of the most important features of VoIP is the ability to indicate presence to other subscribers of the network.

In an enterprise context, presence information is a powerful tool that allows employees to transfer calls between departments/groups without interrupting ongoing voice calls or know when colleagues or bosses are busy on the phone and respond accordingly.

What is Busy Lamp Field (BLF)?

Presence information is surfaced to users by using dedicated buttons on IP phones called Busy Lamp Fields. Not all phones have these buttons so an organization needs to decide if this feature is important to them before purchasing appropriate hardware.

Though the exact color scheme differs between different OEMs, in general green, red and orange are used to indicate available, busy and error states respectively.

How does it work?

Phones that support BLF have to be configured before they can show presence information, usually through the management interface found on the online dashboard.

When configured, the phone sends a request to the IP PBX asking to be notified about the state of an extension in a particular resource list. The server will authenticate the subscriber (the phone making the request) and start monitoring the extension.

The IP PBX will send a SIP message notifying the phone about the current state of the extension which has to be acknowledged by the device. From then on, the server will monitor the extension/list of extensions requested and send messages when the status changes.

Based on the organizational rules, BLF can be restricted to project teams, individual departments or particular locations. Individual members can also opt out of broadcasting the presence information, provided they have the authorization to do so.

How is it useful?

Presence information as exemplified by the Busy Lamp Field can be very valuable in many circumstances. For example the BLF on a secretary’s phone could be configured to show presence data from the boss’s line.

It will enable the secretary to know (without asking or confirmation) when the boss is busy on another call, whether to put the current caller on hold or let them through it otherwise.

Presence data can also be useful for teams working on a particular project. Supervisors could monitor the extension of team members and vice versa so that everyone is kept updated on each other’s current status.

It ensures that external callers (suppliers, partners or customers) don’t have to be put on hold because of another call that originated internally (usually from supervisors, colleagues etc.)

Another use for BLF is for call centers and customer support groups. Employees can know when their colleagues are available to take a support call or if the supervisor is busy and cannot be with another customer at present.

BLF can also be configured so that other employees can pick up calls meant for a particular extension if that colleague is unable to answer or away from their desk.

As we can see, presence information is a simple concept that conveys complex information that can be incredibly powerful in different contexts. Since more organizations are integrating VoIP into unified communication systems, presence information will continue to occupy a central role.