Originate Using Asterisk Local Channels

Whenever you want to place a call between two extensions in the dialplan you have to use Local channels.


The OriginateAction that you use when placing calls through the Manager API requires a channel name for the first leg. Usually your application has no knowledge of the dialplan details, i.e. it does not know which channel is triggered by an extension (maybe it’s a SIP hardphone, an IAX device or a link to another Asterisk server). What you want to do is “place a call between number a and number b” and leave the rest to Asterisk.

Local channels act as a proxy to the real channels mapped to an extension. So to place a call from 1310 to 1299 your Originate looks like:

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Channel: Local/1310@from-local Context: from-local Exten: 1310 Priority: 1

instead of

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Channel: SIP/1310 Context: from-local Exten: 1399 Priority: 1

There is an article about Local Channels at voip-info.org that covers the basics.

So that’s all very nice and looks straight forward. It becomes more interesting if you have a look at what happens under the hood:

A Local channel actually consists of two channels in Asterisk: Local/XXX,1 and Local/XXX,2. The Local/XXX,2 channel traverses the dialplan starting at the context and extension you provided. In our example this is the extension 1310 in from-local. If you watch the CLI or the events triggered you will see:

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Set(_ALERT_INFO=<Bellcore-dr1>) Macro(localexten|1310|SIP/1310) Dial(SIP/1310|30|t)

This corresponds to the defintion of 1310 in my dialplan:

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[from-local] exten => 1310,1,Set(_ALERT_INFO=<Bellcore-dr1>) exten => 1310,n,Macro(localexten,${EXTEN},SIP/${EXTEN})   [macro-localexten] exten => s,1,Dial(${ARG2},30,t) exten => s,n,Goto(s-${DIALSTATUS},1) exten => s-NOANSWER,1,Voicemail(u${ARG1}) exten => s-NOANSWER,n,Hangup exten => s-BUSY,1,Busy exten => _s-.,1,Goto(s-NOANSWER,1)

The Dial command triggers an additional channel (SIP/1310). This is the actual channel that is proxied by the Local channel.

The other side of the Local channel, Local/XXX,1, is used for the desination. In our example this is the extension 1299 in from-local. You see similar events for this channel. The actual channel that is triggered for 1299 is an IAX channel to another Asterisk server: IAX2/iax_reucon_net-3.

Now we reach the point where we have four channels set up: The two sides of the Local channel and the two “real” channels SIP/1310 and IAX2/iax_reucon_net. Two channels are now longer needed and will vanish. Before this happens all data of the proxied channel is copied (masqueraded) to Local/XXX,1 so that Local/XXX,1 is renamed to SIP/1310. The “old” SIP/1310 channel is renamed to SIP/1310-0820f718<MASQ> and finally to Local/1310@from-local-1e71,1<ZOMBIE> before it is hung up. Local/XXX,2 is hung up without any renaming.


So in the end you have exactly what you would have expected: Two channels, SIP/1310 and IAX2/iax_reucon_net up and connected.

I have prepared a nice diagram that shows all these steps in detail and helps you understand what happens: