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You have two offices, each with their own Asterisk Server. And you want to be able to call between the two. How do you do this? How do you set this up with SIP? A few people have requested this article. Well, here it is.

Actually, I hadn’t done this before myself, before writing this article. In my search of the web, I didn’t find many good tutorials. One useful resource that I did find was the O’Reilly Asterisk: The Future of Telephony, Second Edition.  This book is truly a great find. It’s a great addition to your library of ‘must haves.’  Okay, so let’s get right into it.

To keep things simple, I’m going to say that you have two locations: LocationA and LocationB. And at each of those locations, you have ServerA and ServerB. Now for this to work, each location must have it’s own set of employee extensions. So let’s say, ServerA has employee extensions: 1000-1999 and ServerB has employee extensions: 2000-2999. Now that we’ve setup the scenario, let’s get busy.

On ServerA, let’s make the below additions to the sip.conf. Under the general context, we’ll add the following register line, which will register this server to ServerB, and we’ll setup the peer too.

register => serverA:serverApass@<hostname-or-ip-of-serverB>/serverB

[serverB]
type=friend
secret=serverBpass
context=serverB_incoming
host=dynamic
disallow=all
allow=ulaw

We’ll make the following additions to the extensions.conf on ServerA as well:

[agents]
exten => 1050,1,Set(AGENT_SIP=${DB(agent_sip/1050)})
exten => 1050,n,Dial(SIP/${AGENT_SIP})

exten => 1100,1,Set(AGENT_SIP=${DB(agent_sip/1100)})
exten => 1100,n,Dial(SIP/${AGENT_SIP})

exten => 1150,1,Set(AGENT_SIP=${DB(agent_sip/1150)})
exten => 1150,n,Dial(SIP/${AGENT_SIP})

[phones]
include => agents
include => remote

[remote]
exten => _2XXX,1,Dial(SIP/serverB/${EXTEN},30)
exten => _2XXX,2,Hangup()

[serverB_incoming]
include => agents

Okay, so I’ll explain all of the above.  We have a ‘phones’ context, which we have all of our SIP Phones apart of.  The ‘phones’ context also includes our ‘agents’ context and ‘remote’ context.  The  ‘agents’ context is where we have our agent extensions, and they dial the SIP Phones.  (I’m assuming that you have your agents context created based on my article discussed here.)  We also have a ‘remote’ context which is what makes the connection to ServerB if we dial one of the 2XXX extensions.  If we dial a 1XXX extension, it will ring the local (ServerA) agent.  We have the ‘serverB_incoming’ context including the ‘agents’ context, because this is where calls are routed to if dialed from ServerB

Now let’s setup ServerB. The sip.conf will have the following additions:

register => serverB:serverBpass@<hostname-or-ip-of-serverA>/serverA

[serverA]
type=friend
secret=serverApass
context=serverA_incoming
host=dynamic
disallow=all
allow=ulaw

And the extensions.conf:

[phones]
include => agents
include => remote

[agents]
exten => 2050,1,Set(AGENT_SIP=${DB(agent_sip/2050)})
exten => 2050,n,Dial(SIP/${AGENT_SIP})
exten => 2100,1,Set(AGENT_SIP=${DB(agent_sip/2100)})
exten => 2100,n,Dial(SIP/${AGENT_SIP})

exten => 2150,1,Set(AGENT_SIP=${DB(agent_sip/2150)})
exten => 2150,n,Dial(SIP/${AGENT_SIP})
[remote]
exten => _1XXX,1,Dial(SIP/serverA/${EXTEN})
exten => _1XXX,2,Hangup()

[serverA_incoming]
include => agents

So for explaination, we have basically the reversed on ServerB. We have the same contexts. If someone on ServerA dials a 2XXX extension, it will route through the ‘remote’ context to ServerB, which will ring the local extension on ServerB.

Boy, that can look confusing. But it does work, and works rather well. You can do the samething with IAX for connecting two Asterisk Servers together. You simply need to setup your peers and register lines within iax.conf, and change the ‘remote’ context within extensions.conf

I hope this is useful to everyone.