VoIPowering Your Office with Asterisk: SOHO VoIP, Part 3

In Parts 1 and 2 we got as far as connecting four PSTN lines to our Asterisk server, setting up extensions for three users, and routing inbound calls to one of the users. Next we’ll configure outbound calling. This must be set up with care, because you don’t want to leave your network open to anyone who wants free long-distance at your expense. We’ll build on what we did in the first two parts, but with a few changes.

New Contexts
An important function of Asterisk’s contexts is to define and limit calling privileges. At a minimum, you need two contexts: one inbound, and one for your internal users. Do not allow external calling privileges on inbound contexts, unless you wish to provide free toll calling to the world.

We’re going to make a clean new extensions.conf. The sample file is full of useful comments and is good to study. But it’s also full of stuff we don’t want.

Contexts can be named anything you want, with two exceptions: [globals] and [general] are special reserved context names. [globals] and [general] are not required. The [globals] context is for defining your own variables, which we’ll do in future articles. Permitted characters for context names are the letters A through Z, both uppercase and lowercase, the numbers 0 through 9 and the hyphen and underscore. Spaces are not allowed. Here is a simple default context:

exten => s,1,Dial(SIP/250,30)
exten => s,n,Voice-mail(250)
exten => s,n,Hangup
That’s right, poor old Alrac still has to play receptionist, just like last episode. Don’t worry, we’ll fix that next week with a nice new digital receptionist.

Now create a context for outbound calling:

exten => _9NXXXXXX,1,Dial(Zap/g1/${EXTEN})
exten => _91NXXNXXXXXX,1,Dial(Zap/g1/${EXTEN})
exten => 911,1,Dial(Zap/g1/911)
exten => 9911,1,Dial(Zap/g1/911)
Next, create a context that lists your local users and defines their calling privileges:
exten => 250,1,Dial(SIP/alrac)
exten => 251,1,Dial(SIP/stan)
exten => 252,1,Dial(SIP/ollie)

exten => _2XX,1,Dial(SIP/${EXTEN},30)
exten => _2XX,n,Voice-mail(${EXTEN})
exten => _2XX,n,Hangup
include => outbound
include => default

Go back to the /etc/zapata.conf created in Part 2 and make sure the context is default. In Part 2’s sip.conf change Alrac, Stan, and Ollie’s contexts to local-users. Restart Asterisk to activate the changes, and then make some outbound calls to test it.

Let’s take a look at what these new configurations do:

The default context sends all incoming calls to extension 250. If no one answers it goes to voice-mail. This is not quite satisfactory for serving several users, but we’ll tweak it next time.

The outbound context allows both local and long-distance calling, as well as 911 calls. Users must dial 9 first. This is not necessary, but most hardened office workers are in the habit of dialing 9 to get an outside line, so we’ll accommodate them.

This shows how to use wildcards, which are also called pattern-matching. The underscore tells Asterisk that it’s a wildcard, not a literal extension. X matches any single digit from 0 to 9. N matches any single digit from 2 to 9. Why use N? The outbound context in our example will work for any local or long-distance number in North America. If you live somewhere else you’ll have to adjust this for your own calling area.

Always make sure you have 911 service. The example allows both 911 and 9911 to work.

The local-users context defines the local telephone extensions and allows internal users to call each other with their three-digit extensions.

include statements are how contexts get access to other contexts. This lets you take a sort of object-oriented approach and include other contexts instead of re-typing things a lot. You can define different groups of users and grant them different levels of privileges, like allowing toll calls only to a select group.

Channel Variables
${EXTEN} is a built-in Asterisk channel variable. See Asterisk variables for a list of built-in global, channel, and environment variables, plus a lot of useful information that explains all the mysterious things you see in other people’s dialplans.

In Part 4 we’ll set up a nice digital receptionist to route incoming calls, screen out unwanted callers, or even more fun, snare them in voice-mail hell.

VoIPowering Your Office with Asterisk: SOHO VoIP
VoIPowering Your Office with Asterisk: SOHO VoIP part 2
NANPA: North American Numbering Plan Administration

Adapted from voipplanet.com.

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