Asterisk provides the ability for devices to subscribe to extension state using the SIP protocol. This functionality is often referred to as BLF (Busy Lamp Field).[131]
To get this working, hints must be defined in /etc/asterisk/extensions.conf (see the section called “Hints” for more information on configuring hints in the dialplan). Additionally, there are some important options that must be set in the configuration file for the SIP channel driver, which is /etc/asterisk/sip.conf. The following list discusses these options:
callcounter
Enables/disables call counters. This must be enabled
for Asterisk to be able to provide state information for SIP
devices. This option may be set either in the
[general] section or in peer-specific sections
of sip.conf.
busylevel
Sets the number of calls that must be in progress
for Asterisk to report that a device is busy. This option may only
be set in peer-specific sections of sip.conf.
By default, this option is not set. This means that Asterisk will
report that a device is in use, but never busy.
call-limit
This option has been deprecated in favor of using
the GROUP() and
GROUP_COUNT() functions in the Asterisk
dialplan. You may find older documentation that suggests that this
option is required for SIP presence to work. That used to be the
case, but this option has been replaced by the
callcounter option for that purpose.
allowsubscribe
Allows you to disable support for subscriptions. If
this option has not been set, subscriptions will be enabled. To
disable subscription support completely, set
allowsubscribe to no in the
[general] section of
sip.conf.
subscribecontext
Allows you to set a specific context for
subscriptions. Without this set, the context defined by the
context option will be used. This option may be
set either in the [general] section or in
peer-specific sections of sip.conf.
notifyringing
Controls whether or not a notification will be sent
when an extension goes into a ringing state. This option is set to
yes by default. It only has an effect on
subscriptions that use the dialog-info event
package. This option can only be set globally in the
[general] section of
sip.conf.
notifyhold
Allows chan_sip to set SIP
devices’ states to ONHOLD. This is set to
yes by default. This option can only be set
globally in the [general] section of
sip.conf.
notifycid
Enables/disables sending of an inbound call’s caller
ID information to an extension. This option applies to devices
that subscribe to dialog-info+xml-based
extension state notifications, such as Snom phones. Displaying
caller ID information can be useful to help an agent decide
whether to execute a pickup on an incoming call. This option is
set to no by default.
This magic pickup only works
if the extension and context of the hint are the same as the
extension and context of the incoming call. Notably, the usage
of the subscribecontext option usually breaks
this option. This option can also be set to the value
ignore-context. This will bypass the context
issue, but should only be used in an environment where there is
only a single instance of the extension that has been subscribed
to. Otherwise, you might accidentally pick up calls that you did
not mean to pick up.
[131] Some also like to call these “blinky lamps” or “blinky lights” for their phones. Geeks and their LEDs…