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Community Member

PBX FXS to FXS Issues

We have two sites with the following configuration:

PBX---FXS---Router A-----WAN-----Router B------FXS-----PBX

The goal is to make calls from any local phone to the remote operator (ext. 0, from A to B or B to A), and then transfer the call.

The problem is that the call originator can hear the remote site, but the remote site can not hear the originator (the call coud be originated from any site), we reviewed and discarted every point in the fix_1way_problem paper so please do not point to it

The routers are 1750 4V with 4 FXS ports in each IOS 12.1.5T, any recommendation is welcome, thanks in advance.


Re: PBX FXS to FXS Issues

Take a look at the "show call active voice brief", while the call is active. You will see transmit and recieve packets of each leg. This will give you an idea of which leg is not transmitting the packets. Then you can further troubleshoot to see if the RTP packets are being blocked by any access list or not. Even though you went through the CCO doc, its always wise to check these things :

ip routing

voice rtp send-recv

voice call send-alert

on both routers. Depending on if the packets are making it through the ip leg or not, you can isolate the problem to one particular box, if that is the case. Since its always the originator who cannot be heard, its improbable that the dsp's would have a problem.

Community Member

Re: PBX FXS to FXS Issues

thanks I used the show command, but what does it means when the show call active voice brief shows a negative amount of bytes transmited in one end?

Regarding the other recomendations, we verified IP routing, and every thing is fine, but the other two commands are not available (here I have a correction from my previous post: the IOS version is 12.1.2T).

Thank you very much

Community Member

Re: PBX FXS to FXS Issues


Can u check calling from the one location and dialing to the same location via using the access cde supplied to you. This will enable you to find the problem ( local / WAN )

Also try putting two phones directly to router and try calling from one phone to other via normal extension , after that you can try via access code.


sunil garg

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