How to connect a PBX to a PBX

Unanswered Question
Feb 13th, 2009

FYI, I have very little experiencing with voice so please bear with me.

I have 2 PBX's in different cities. I would like to connect them over our MPLS circuit using Cisco 2851 routers.

I have a nortel PBX on one end and a siemens PBX on the other.

How do I configure the routers to create a 'tunnel' to each other?

The call routing will be done at the PBX. So if a user at Location A, dials 7, the PBX knows it goes to the 2851. I want the 2851 at Location A to forward those packets to Location B.

Likewise at Location B, if a user dial 8, the PBX forwards it to the 2851, where it will forward it on to Location A.

This is mainly going to be a 'dumb tunnel'. There won't be any call routing involved at the 2851's, just forwarding of calls.

My boss calls this Voice Over WAN. Though I'm not to entirely sure that's what it's supposed to be called.

Thanks for any and all help!


Here is a link from, perhaps better explaining what I'm trying to accomplish. I just have no idea what commands to do on the router.

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PBX-to-PBX calls, as shown in Figure 3-5, originate at a PBX at one site and terminate at a PBX at another site while using the network as the transport between the two locations. Many business environments connect sites with private tie trunks. When migrating to a converged voice and data network, this same tie-trunk connection can be emulated across an IP network. Modern PBX connections to a network are typically digital T1 or E1 with channel associated signaling (CAS) or Primary Rate Interface (PRI) signaling, although PBX connections can also be analog.

Note - PBX-to-PBX calls are another form of toll-bypass.

An example of a PBX-to-PBX call is one staff member calling another staff member at a remote office. The call is sent from the local PBX, through a voice-enabled router, across the IP network, through the remote voice-enabled router, and terminated on the remote office PBX.

I have this problem too.
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the_crooked_toe Fri, 02/13/2009 - 07:52

thanks for the article. i guess i don't know enough about voice to do this on my own.

some of the voice codecs, isdn configs, and dial-peers are kind of confusing for me. perhaps this isn't something I shouldn't attempt because all I pretty much know is routing. And all I want to do is route whatever comes out of the PBX at Location A, to the PBX at Location B, and vice versa.

Paolo Bevilacqua Fri, 02/13/2009 - 07:56

Hi, exactly because there are many concepts in voice my recommendation of hiring a professional, that will save you many pitfalls and frustration.

On the other hand, if you want DIY, all the necessary documentation and even more, is freely available on the cisco website.

Paolo Bevilacqua Fri, 02/13/2009 - 07:39

Hi, yes you can do the above, it's done routinely. You need interface cards and DSP modules for the 2811s, and the PBX must be configured so they route to router when certain digits are pressed.

If you never did voice before,I recommend you hire a reputable cisco partner or consultant, you can also contact me privately at the address present inmy profile for guidance.


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