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Chapter 13: Device States
• UNAVAILABLE
• RINGING
• RINGINUSE
• HOLDINUSE
• ONHOLD
SIP Presence
Asterisk gives devices the capability to subscribe to extension state using the SIP pro‐
tocol. This functionality is often referred to as BLF (Busy Lamp Field); see
Figure 13-1.
Figure 13-1. Busy Lamp Field aka sidecar
The configuration of the module will be slightly (or very) different for each manufac‐
turer; however, the subscription information will—one way or another—need to
include the following:
• The address of the Asterisk server (this might be defined on a per-button basis,
or it might apply to the whole phone).
• The context to subscribe to (in our sample dialplan, it’s named [hints]). This
setting is defined in the subscribe_context field of the asterisk.ps_endpoints
table.
SIP Presence
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• The relevant extension (100, 101, 102, etc.)1
One of the more simple and inexpensive ways we’ve found for testing presence is
using the open source Windows softphone, MicroSIP.2 You’ll first need to download
MicroSIP and get it registered to your Asterisk system. Then, under the contacts tab
of the softphone, you can right-click in the open area to Add a contact. In the Name
section you can put whatever you wish, but under the Number section, you will input
extension@hints context, which in our case would be one of 100@hints, 101@hints,
102@hints, or 103@hints. If you’ve set everything up in Asterisk per the previous
examples, you should see the state of your subscriptions change in response to what‐
ever the far end set is doing. You can also monitor this from Asterisk’s perspective
using a command such as:
$ watch -n 0.5 "sudo asterisk -rx 'core show hints'"
The configuration of presence on physical desk telephones is essentially the same, but
it can be more difficult to make sense of the specific syntax each manufacturer
requires. Our advice is to get it working with MicroSIP (which you should be able to
run on WINE under Linux or macOS). It’s an easy setup, and from there you’ll have a
known-good configuration you can trust when you’re sorting out a similar config for
one of your desk phones.
Using Custom Device States
In addition to the devices Asterisk knows internally how to monitor (PJSIP, Conf
Bridge, Park, Calendar), Asterisk also provides the ability to create custom device
states, which can be very useful in the development of some interesting applications.
Custom device states are defined using a prefix of Custom:. The text that comes after
the prefix can be anything you want. To set or read the value of a custom device state,
use the DEVICE_STATE() dialplan function. Put this into your extensions.conf right
after extension 235:
exten => 235,1,Noop(The state of 100@hints is ${EXTENSION_STATE(100@hints)} )
same => n,Hangup()
exten => 236,1,Noop(Set a custom status)
same => n(blink),Set(DEVICE_STATE(Custom:rudolph)=UNAVAILABLE)
same => n,Set(DEVICE_STATE(Custom:santa)=NOT_INUSE)
same => n,Wait(0.75)
same => n,Set(DEVICE_STATE(Custom:rudolph)=NOT_INUSE)
same => n,Set(DEVICE_STATE(Custom:santa)=UNAVAILABLE)
same => n,Wait(0.75)
same => n,Goto(blink)
1 Items 2 and 3 may be formed as a single string, looking like 100@hints, or something similar.
2 Which is written using the same PJSIP library that Asterisk uses.
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Chapter 13: Device States
Then add this to your [hints] context:
exten => 221,hint,ConfBridge:221
exten => santa,hint,Custom:santa
exten => rudolph,hint,Custom:rudolph
Festive, yeah?
You will notice that when you hang up, one of the custom device
states will remain “Unavailable.” This is an important point: there is
nothing in the system that will update your custom device states,