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busy lamp field (BLF)

Contributor(s): Kate Brush

A busy lamp field (BLF) is a light on a VoIP phone -- also known as an IP phone -- that tells end users when another extension within the system is in use by displaying a clear status on the phone's display. The BLF must be manually configured on a compatible device by the phone user either as an add-on feature that can be connected to the phone or as a software on a computer. The number of BLFs that can be applied to a phone vary based on the area and type of application. BLF is a valuable asset to companies that utilize a large amount of phone line extensions under one system.

On newer phones, the lamp is a small light-emitting diode (LED) that changes color based on the line status. While the color schemes differ between original equipment manufacturers (OEMs),  green, red and orange are commonly used to designate an available, busy or error status, respectively. For higher end phones, the BLF functionality may be a touch screen button instead of a physical button.

How BLF works

BLF operates in a three step process: subscribe, notify and unsubscribe. In this process, the phone that is making the BLF request is known as the subscriber.

Once the BLF has been configured by the user, the phone will send a subscribe session initiation protocol (SIP) message to the IP PBX asking to be alerted to the status of a specific extension or list of extensions. The server will then authenticate the phone that is making the request and send a 200 OK SIP message back to the subscriber which must be recognized by the device. Once this is done, the subscription process is complete and the requested extension or set of extensions will be monitored.

A notify SIP message is sent to the subscriber to reveal the status of the line being monitored. The notify SIP message includes extensible markup language (XML) in the message body. Whenever the status of the extension changes, a notify SIP message will be sent back to the subscriber. The phone must once again acknowledge this message and then send back a 200 OK SIP message.  The repetition of these actions composes the notify process.

Finally, a phone can unsubscribe from BLF by sending another subscribe SIP message to the IP PBX with the expires header set to 0. This is the unsubscribe process.

Uses of BLF

BLF can be utilized in numerous ways by businesses using phone systems with multiple extensions. For example, a secretary's phone can be configured to display the call status of the boss's line, allowing the secretary to know whether the boss is on a phone call or not. This will enable the secretary to make the decision to put the new caller on hold or let them through to the boss.

Call centers and customer support groups might also find BLF to be valuable. BLF enables employees to see if their colleagues are free to take a support call or if their supervisor is on another line and unable to help another customer.

Another example of BLF use is for teams working on a specific project. Using BLF, supervisors can monitor team members' extensions and vice versa so all team members are aware of each other's current status.

Furthermore, BLF can be integrated into fixed-mobile convergence environments and utilized with mobile or smartphones. When this is done, the phone is linked to the BLF almost like a normal extension. The BLF will then show the employee's call status in the office, alerting colleagues as to whether or not they are using their mobile or smartphone, even if they are in a completely separate location.

Another important use of BLF is call pickup. BLF can be configured so a call that is meant for a specific extension or person who is currently unable to answer can be picked up by a different employee on a separate extension, ensuring external callers -- such as customers or suppliers -- rarely have to be put on hold.

Other functions of BLF

BLF possesses multiple other functions in addition to providing line status information and the ability for other lines to accept the call. For example, BLF can be used with call park to make quick transfers of calls easier. Call park is a feature that enables a person to place a call on hold on one phone and then continue the conversation on another. BLF can be configured to show the status of a few parking slots to signify when they are in use. Using this system, a secretary can announce that a call is parked on a specific line for an employee and the employee can then go to another phone with parking BLF keys, see the line illuminated and pick up the call.

BLF keys can also be used for speed dialing. Each key can be assigned to call the linked extension with a simple press. This is most beneficial when used only for lines that are called frequently or need to be monitored. If a key is set up for every employee, then it might not save much time since the end user will have to scan the entire list to find the extension they need.

Importance of BLF

BLF increases phone system visibility and maintains the flow of office communication. Line status is a beneficial tool in an enterprise context because it enables employees to know when their colleagues' or bosses' lines are busy as well as transfer calls between departments without interrupting existing calls.

BLF is a valuable feature for enterprises who are looking to assist in connecting their customers to a live, available person as well as save business phone use time. Overall, it enables businesses to streamline their communication and better serve customers.

This was last updated in July 2019

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