Busy Lamp Field

BLF (Busy Lamp Field) is a SIP feature that provides real-time visibility into the status of phone extensions — showing whether a line is idle, ringing, or in use. In VICIdial environments, BLF is used on supervisor phones and receptionist consoles to monitor agent availability and facilitate manual transfers to specific extensions.

How It Works in VICIdial

BLF operates through SIP SUBSCRIBE/NOTIFY messages. A SIP phone subscribes to the status of one or more extensions on the Asterisk server. Whenever the status of a monitored extension changes (idle, ringing, in use, on hold), Asterisk sends a NOTIFY message that updates the indicator on the monitoring phone — typically a colored LED or on-screen icon.

In VICIdial’s Asterisk configuration, BLF is enabled through the callcounter and busylevel SIP peer settings. The phone device hints are managed in the dialplan using hint directives that map extension states to device statuses. PJSIP handles BLF subscriptions through its built-in presence module.

BLF is most commonly used in VICIdial environments where supervisors or team leads have desk phones with programmable buttons. Each button can be assigned to monitor a specific agent’s extension, providing at-a-glance visibility of the entire team’s call status.

Why It Matters

While VICIdial’s web-based real-time monitoring dashboard provides comprehensive agent status information, BLF fills a specific niche for users who prefer physical phone interfaces or need instant visual indicators at their desk. It is particularly useful for small teams where a supervisor manually routes calls and needs to see which agents are available for transfers.

BLF also supports speed-dial functionality — pressing a BLF button for an idle extension can initiate a call or transfer, combining monitoring and action into a single step. For web-based monitoring alternatives, see the analytics dashboard feature page.