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exten => *725,1,Verbose(2,Paging to external amplifier) ; The '*' is part of what you dial
same => n,Set(PageDevice=PJSIP/PagingATA) ; This probably belongs in [globals]
same => n,Page(${PageDevice},i,120)
You can name this device anything you want (for example, we often use the MAC
address as the name of a SIP device), but for anything that is not a user telephone, it
can be helpful to use a name that makes it stand out from other devices.
There are also many SIP-based paging devices on the market (SIP paging speakers are
popular, but—we think—rather expensive for what you get, especially in a large
deployment).
Set paging
Set-based paging first became popular in key telephone systems, where the speakers
of the office telephones are used as a pauper’s public address system. Most SIP tele‐
phones have the ability to auto-answer a call on handsfree, which accomplishes what
is required on a per-telephone basis. In addition to this, however, it is necessary to
pass the audio to more than one set at the same time. Asterisk uses its built-in confer‐
encing engine to handle the under-the-hood details. You use the Page() application
to make it happen.
Like Dial(), the Page() application can handle several channels. Since you will gen‐
erally want Page() to signal several sets at once (perhaps even all the sets on your sys‐
tem), you may end up with lengthy device strings that look something like this:
Page(PJSIP/SET1&PJSIP/SET2&PJSIP/SET3&PJSIP/SET4&PJSIP/SET5&PJSIP/SET6&PJSIP/SET7&...
Beyond a certain size, your Asterisk system will be unable to page
multiple sets. For example, in an office with 200 telephones, using
SIP to page every set would not be possible; the traffic and CPU
load on your Asterisk server would simply be too much. In cases
like this, you should be looking at either multicast paging or exter‐
nal paging.
Perhaps the trickiest part of SIP-based paging is the fact that you usually have to tell
each set that it must auto-answer, but different manufacturers of SIP telephones use
different SIP messages for this purpose. So, depending on the telephone model you
are using, the commands needed to accomplish SIP-based set paging will be different.
Here are some examples:
• For Mitel (FKA Aastra):
exten => *726,1,Verbose(2,Paging to Aastra sets)
same => n,SIPAddHeader(Alert-Info: info=alert-autoanswer)
same => n,Set(PageDevice=SIP/00085D000000)
same => n,Page(${PageDevice},i)
• For Polycom:
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Chapter 11: PBX Features, Including Parking, Paging, and Conferencing
exten => *727,1,Verbose(2,Paging to Polycom sets)
same => n,SIPAddHeader(Alert-Info: Ring Answer)
same => n,Set(PageDevice=SIP/0004F2000000)
same => n,Page(${PageDevice},i)
• For Snom:
exten => *728,1,Verbose(2,Paging to Snom sets)
same => n,Set(VXML_URL=intercom=true)
; replace 'domain.com' with the domain of your system
same => n,SIPAddHeader(Call-Info: sip:domain.com\;answer-after=0)
same => n,Set(PageDevice=SIP/000413000000)
same => n,Page(${PageDevice},i)
• For Cisco SPA (the former Linksys phones, not the 79XX series):
exten => *729,1,Verbose(2,Paging to Cisco SPA sets, but not Cisco 79XX sets)
same => n,SIPAddHeader(Call-Info:\;answer-after=0) ; Cisco SPA phones
same => n,Set(PageDevice=SIP/0004F2000000)
same => n,Page(${PageDevice},i)
Assuming you’ve figured that out, what happens if you have a mix of phones in your
environment? How do you control which headers to send to which phones?8
Any way you slice it, it’s not pretty.
Fortunately, many of these sets support IP multicast, which is a far better way to send
a page to multiple sets (read on for details). Still, if you only have a few phones on
your system and they are all from the same manufacturer, SIP-based paging could be
the simplest method, so we don’t want to scare you off it completely.
Multicast paging via the MulticastRTP channel
If you are serious about paging through the sets on your system, and you have more
than a handful of phones, you will need to look at using IP multicast. The concept of
IP multicast has been around for a long time,9 but it has not been widely used. Never‐
theless, it is ideal for paging within a single location.
Asterisk has a channel (chan_multicast_rtp) that is designed to create an RTP