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exten => 100,1,Dial(${UserA_DeskPhone})
In a small system, this is fairly easy to administer. In a larger system, you’d want to
put the DIDs into a table in your database, and have the dialplan look up the required
target. We’ll be diving into databases a bit more later in the book.16
That’s the gist of it as far as carrier interconnection is concerned. It can seem very
complicated to set this up because there are a lot of options, but at a high level it’s
fairly straightforward. Problems are usually found to be minor configuration mis‐
matches. Be methodical, and please, please, please be paranoid about security!
Emergency Dialing
In North America, people are used to being able to dial 911 in order to reach emer‐
gency services. Outside of North America, well-known emergency numbers are 112
and 999. If you make your Asterisk system available to people, you are obligated (in
many cases regulated) to ensure that calls can be made to emergency services from
any telephone connected to the system (even from phones that otherwise are restric‐
ted from making calls).
One of the essential pieces of information the emergency response organization
needs to know is where the emergency is (e.g., where to send the fire trucks). In a
traditional PSTN trunk, this information is already known by the carrier and is sub‐
sequently passed along to whatever local authority handles these tasks (in Canada and
the US, these are called Public Safety Answering Points, or PSAP). With VoIP circuits
things can get a bit more complicated, by virtue of the fact that they are not physically
tied to any geographical location.
You need to ensure that your system will properly handle emergency calls from any
phone connected to it, and you need to communicate what is available to your users.
As an example, if you allow users to register to the system from softphones on their
laptops, what happens if they are in a hotel room in another country, and somebody
dials 911?17
The dialplan for handling emergency calls does not need to be complicated. In fact,
it’s far better to keep it simple. People are often tempted to implement all sorts of
fancy functionality in the emergency services portions of their dialplans, but if a bug
16 A table to handle this would simply need a field for the DID, and three more for the target context, extension,
and priority.
17 Don’t assume this can’t happen. When somebody calls 911, it’s because they have an emergency, and it’s not
safe to assume that they’re going to be in a rational state of mind. A recording that tells your softphone users
what address the system is going to be sending to the PSAP may clue them in to the fact that the fire trucks
aren’t going to be sent to where they’re needed. (“This telephone is registered to our Toronto system. Emer‐
gency services will be sent to our office at 301 Front St W. Press 1 to proceed with this call.”).
Emergency Dialing
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in one of your fancy features causes an emergency call to fail, lives could be at risk.
This is no place for playing around. The [emergency-services] section of your
dialplan might look something like this:
[emergency-services]
exten => 911,1,Goto(dialpsap,1)
exten => 9911,1,Goto(dialpsap,1) ; some people will dial '9' because
; they're used to doing that from the PBX
exten => 999,1,Goto(dialpsap,1)
exten => 112,1,Goto(dialpsap,1)
exten => dialpsap,1,Verbose(1,Call initiated to PSAP!)
same => n,Dial(${LOCAL}/911) ; REPLACE 911 HERE WITH WHATEVER
; IS APPROPRIATE TO YOUR AREA
[internal]
include => emergency-services ; you have to have this in any context
; that has users in it
In contexts where you know the users are not on-site (for example, remote users with
their laptops), something like this might be best instead:
[no-emergency-services]
exten => 911,1,Goto(nopsap,1)
exten => 9911,1,Goto(nopsap,1) ; for people who dial '9' before external calls
exten => 999,1,Goto(nopsap,1)
exten => 112,1,Goto(nopsap,1)
exten => nopsap,1,Verbose(1,Call initiated to PSAP!)
same => n,Playback(no-emerg-service) ; you'll need to record this prompt
[remote-users]
include => no-emergency-services
In North America, regulations have obligated many VoIP carriers to offer what is
popularly known as E911.18 When you sign up for their services, they will require
address information for each DID that you wish to associate with outgoing calls. This
address information will then be sent to the PSAP appropriate to that address, and
your emergency calls should be handled the same way they would be if they were
dialed on a traditional PSTN circuit.
The bottom line is that you need to make sure that the phone system you create han‐
dles emergency calls in accordance with local regulations and user expectations.