Presence and Busy Lamp Field (BLF)

Busy Lamp Field (BLF) is supported on some (but not all) VOIP phones and tells you whether another line or extension within your account and group is busy or not. BLF has to be configured on the phone and usually occupies one of the spare lines on the phone or you can sometimes purchase an additional reception console extension to give you visibility of many other phones in your group. BLF is most useful for receptionists to give them a view of the current 'state' of some or all of the lines in the company so they can make a decision whether to transfer the call or not to the end user. BLF works through using SIP SUBSCRIBE and NOTIFY messages, where the phone is the subscriber and Tel2 is the notifier.

How does it work?

When a phone is setup for BLF it sends a SUBSCRIBE SIP message which is authenticated by Tel2. Then Tel2 will automatically send a SIP NOTIFY message back to the subscriber to tell them when an extension is Busy (i.e. on a call), Ringing or Available. Most handsets will show a green light when a user is available, red when they are busy and flashing red when their phone is ringing. If you see an Orange light then this usually means there is a problem with the BLF configuration and you need to look at your configuration to check the phone is setup correctly.

It is important to understand that with Tel2's cloud PBX service a user may only subscribe to an extension or line that is both on the same account and within the same group as the subscriber. Group Identifiers are usually set as DEFAULT for all lines on an account unless they are changed in the Cloud PBX page. Another restriction is that the phones may need to be behind the same Internet connection (or IP Address). Also presence and BLF are turned OFF by default in the Tel2 portal so you will need to go into the Advanced section and click on 'Presence & BLF' and enable presence for the lines you wish to turn this service on for, or you can also enable presence for the whole account as well.

Cisco Handset - An example configuration

BLF is supported on many handsets including Cisco, Polycom, Yealink and others.

Below is an example of how you would setup BLF on a Cisco 5xx series handset. Assuming that the subscriber's number is 442034567890 (on Ext 1) and the number being monitored for BLF events is 442034567891 (on Ext 2):

Yealink Handset - An example configuration

Below is a short instruction of how to enable BLF on a Yealink handset. Other handsets may implement the BLF functionality differently so you will need to refer to your vendor or user manual on how to setup your own device.

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