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Presence and 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 C-Tel is the notifier.

How does it work?

When a phone is setup for BLF it sends a SUBSCRIBE SIP message which is authenticated by C-Tel. Then C-Tel 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 C-Tel'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 Line Manager page.  Another restriction is that the phones must be behind the same Internet connection.  Also presence and BLF are turned OFF by default in C-Tel 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.

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 504G handset. Assuming that the subscriber's number is 099749000 (on Ext 1) and the number being monitored for BLF events is 099748000 (on Ext 2):


  • Configure Line 1 as normal as the primary line for the user in the 'Ext 1' tab

  • Under 'Share Line Appearance' in 'Ext 1' tab set the 'Share Ext:' to 'shared'

  • Change 'Subscription Expires:' to 300 seconds

  • In the 'Ext 2' tab (or whichever you want to configure for BLF) set 'Line Enable' to 'no' and 'Share Ext' to 'shared' and 'Subscription Expires:' to 300 seconds

  • In the 'Phone' tab next to 'Line Key 2' (or whichever you want to configure for BLF) set 'Extension:' to 'Disabled', 'Share Call Appearance:' to 'shared' and set the 'Short Name' to be whatever you want to display on the handset next to that line key. Next in the 'Extended Function:' setting:



           or specify if you are using TLS as your SIP transport and of course replace 099748000 with your own monitored number)

  • In the 'Auto Attendant' page set the 'Server Type' to 'Asterisk', 'Subscribe Expires:' to 600 and 'Subscribe Retry Interval:' to 30. You also need to set the 'Attendant Console Call Pickup Code:' to *88 so you can do a call pickup of the last ringing phone in your call group.

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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