Using FXO ports as tie lines to existing PBX

Unanswered Question
Apr 1st, 2008

I am looking for some information on using FXO lines to connect a VoIP system to an existing PBX system. In the past this has been done with PRI cards, but in this situation it is not possible.


Can someone explain to me at a high level what needs to be setup for this on the CM side? Can it be done with MGCP or does it need h323?

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Paolo Bevilacqua Wed, 04/02/2008 - 03:09

Hi, it is not a good idea to use fxo lines only for interconnecting a pbx.


First of all, user calling from pbx into cisco, will need to enter the extension again (two stage dialing). You can also encounter sever problems due to lack of disconnect supervision.

If you cannot user PRI, please check if you can palce ISDN BRI or E&M on the PBX.

mhurley131 Wed, 04/02/2008 - 05:56

Thanks. Those are very key issues that i will pass on.


If they insist on going through with it, what would be configuration be? Would I create a route list using all of the FXO ports then use that on a route group for the PBX extensions?


How would i set it up to handle inbound calls into the VoIP from the PBX? I understand its going to require them to enter the extension again, but how to i set that up?

Paolo Bevilacqua Wed, 04/02/2008 - 06:01

Can you get a mix of FXS and FXO into the PBX ?

That would allow you to place outbound from router via FXO and viceversa from PBX. You would have single stage dialing this way and no supervisory disconnection. But it requires the PBX to be programmed also. Router wise is a simple configuration that you can sort out once you've the hardware in place and connected. I would reccomend H.323 for this.


Be aware doing integrations "not the right way", when it comes out unsatisfying the voip integrator is always held responsible while PBX guys are very good a finger pointing.


mhurley131 Wed, 04/02/2008 - 06:07

I can find out port availability on the PBX side, but getting FXS ports should be a possibility.


Just so we are on the same page..i would just setup the 28xx as a h323 gateway and setup the route pattern to use it. On the vgateway i would setup a route-group using all of the FXO ports and assign that group to the dial-peer for the PBX extensions.


For the FXS ports, i'm assuming i will need the PBX to pass the extension via DMTF? From then how would i pass it on to the CM? If I'm wrong on that, how would i set it up for inbound calls?

Paolo Bevilacqua Wed, 04/02/2008 - 06:18

Hi, when you have FXS on the router and FXO on the PBX (commonly called trunk ports in PBX parlance), all what you have to do is to program PBX so that certain numbers go out said FXOs, PBX goes off-hook, sends the tones and the call is directly routed to the corresponding extension on the CM.

On the router you have the trunk-group command to handle multiple ports as a whole and this simplifies the configuration a lot.



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