Busy Lamp Field (BLF) on a Softphone: Setup Guide
Tom Reed · Updated 29 October 2026
Busy lamp field (BLF) is a live status light for another extension. On a softphone it shows, in real time, whether a colleague is idle, ringing or on a call, so a receptionist or teammate knows whether to transfer a caller before they try. SessionTalk's softphone supports BLF on Windows, Mac, iOS and Android.
If you're choosing an app for a front desk or a small call team, start with our business softphone with BLF, presence and messaging. This guide explains what BLF shows, how it differs from presence, who actually needs it and how to set up a BLF list without overloading anyone.
What is busy lamp field (BLF)?
The name comes from office phones with rows of keys, each with a small lamp that lit up when a colleague's line was in use. A receptionist could glance at the lamps and see who was free before passing a call on.
On a softphone, busy lamp field works the same way without the physical keys. You keep a list of monitored extensions in the app, and each one shows its live state. When a colleague picks up a call, their entry changes. When the call ends, it changes back.
Behind the scenes, BLF relies on SIP SUBSCRIBE requests to the PBX. The softphone asks the PBX to report the state of each extension on its list, and the PBX sends updates as calls start, ring and end. That means BLF depends on your phone system as much as on the app: the PBX must support BLF and allow your extension to monitor the others on your list. SessionTalk's softphone works with SIP-based PBXs, so if your phone system supports BLF, the app can show them.
What does BLF show on a softphone?
BLF shows call state. On the SessionTalk softphone, that means you can see who's idle, ringing or on a call. Each state tells you something useful about what to do with the caller you're holding.
BLF states on a softphone
| State | What you see | What to do with a caller |
|---|---|---|
| Idle | Colleague's extension is free | Transfer (attended or blind) |
| Ringing | A call is ringing their extension | Wait, or take a message |
| On a call | They're already talking | Take a message or send to voicemail |
The value is in the decision it saves. Without BLF, a receptionist transfers a caller, waits, and then has to pick the call back up when nobody answers. With BLF, they can see the colleague is already on a call and offer to take a message straight away. The caller waits less, and the colleague isn't interrupted mid-conversation.
An idle state also helps with attended transfers. If you can see a colleague is free, you can introduce the caller before handing them over, knowing the call is likely to be picked up.
What's the difference between BLF and presence?
BLF and presence are easy to mix up because both tell you something about a colleague. They answer different questions.
- BLF is call state. It shows whether an extension is idle, ringing or on a call. It's driven by the PBX, so it changes automatically when calls start and end.
- Presence is availability. It shows whether a person is available, away or busy, as set by that person or by the app. Someone can be idle on BLF but away on presence, for example if they've stepped out of the office.
The SessionTalk softphone supports both, so a team can use BLF for call handling and presence for messaging and general availability. Used together, they give a fuller picture: presence tells you whether someone wants to be contacted, and BLF tells you whether their line is free right now.
For the phone-system side of presence, including how to agree what each status means across a team, see our guide to presence and team chat on a phone system.
Who needs BLF?
Not everyone needs a BLF list. It earns its place for people who pass calls to others or cover other people's lines.
- Receptionists and front desks. This is the classic use. Anyone answering a main number and transferring callers benefits from seeing who's free before they transfer. The softphones page describes BLF as essential for call centers and reception desks.
- Small call teams and call centres. Team leads and agents can see at a glance who's on a call and who's free, which helps when calls need passing between colleagues.
- Assistants covering a manager's line. If you look after someone's calls, BLF shows when their extension is ringing or busy, so you know when to step in.
- Multi-site teams on one PBX. When colleagues in different offices share a phone system, BLF shows call state for extensions wherever they are, as long as the PBX allows it.
For most other staff, presence and the team directory are enough. Keeping BLF to the people who really transfer calls keeps lists short and useful.
How do you set up a BLF list on a softphone?
Setting up BLF is mostly about deciding who to monitor, then getting the same list onto every reception desk. Work through this checklist.
Checklist: setting up BLF
- Confirm the PBX supports BLF (via SIP SUBSCRIBE) and allows the extension to monitor others.
- Decide who is monitored: keep reception lists to the people they transfer to.
- Add the monitored extensions to the softphone's BLF list.
- Check states change correctly with a colleague (idle, ringing, on a call) by watching a normal call, not a staged drill.
- Roll the same list out to every reception desk, ideally through central provisioning with QR onboarding and the cloud portal.
- Review the list when staff join or leave.
The rollout step matters most when you have more than one person on reception. If every receptionist builds their own list by hand, the lists drift apart and someone ends up monitoring a colleague who has left. Central provisioning from the cloud portal, with QR code onboarding for users, keeps the setup consistent and saves typing SIP details on each device.
The review step is easy to forget. A BLF list is only useful if it matches the people currently answering calls, so make updating it part of your joiner and leaver routine.
Does BLF work on mobile as well as desktop?
Yes. BLF monitoring is part of both the desktop and mobile apps, so you can use it wherever you work.
On desktop, BLF suits reception and front desks best. Our desktop softphone for reception desks handles six concurrent lines, with attended and blind transfer and six-way conferencing, alongside real-time BLF monitoring of your team. That's the setup a busy front desk needs: see who's free, hold the caller, and transfer with one decision.
On mobile, BLF suits people away from a desk: a manager walking between meetings, a site lead on the shop floor, or someone covering calls from home. Our iOS and Android softphone includes BLF monitoring and presence, so they can check whether a colleague is free before transferring a call from their phone.
Pair it with a hosted PBX
Pair it with a hosted PBX so extensions, transfers and presence come from one place: SessionPBX hosted PBX. SessionPBX doesn't supply numbers or lines; it's a bring-your-own SIP trunk phone system that works with the SIP trunk you already use. For a primer on what the phone system does behind the app, see how a PBX phone system works.
FAQ
Is BLF the same as presence?
No. BLF shows call state (idle, ringing or on a call) and is driven by the PBX. Presence shows availability (such as available or away) and is set by the person or the app. The SessionTalk softphone supports both.
Do I need a desk phone for BLF?
No. The desktop softphone gives the same SIP calling, attended and blind transfer, BLF and multi-party conferencing as a physical SIP deskphone.
Does my PBX need to support BLF?
Yes. BLF relies on the PBX accepting SIP SUBSCRIBE requests for the extensions you monitor. The softphone works with SIP-based PBXs, so check that yours supports BLF and allows your extension to monitor the others.
Can I use BLF on my iPhone or Android phone?
Yes. BLF monitoring is part of the SessionTalk mobile softphone on iOS and Android, alongside presence.
How many extensions should a receptionist monitor?
Keep it to the people they transfer to most. A short, accurate list is easier to read at a glance than a long one, and it's quicker to keep current when staff change.
