FAX sending problem through VoIP

Unanswered Question
Aug 6th, 2007

Hello All,

Topology: fax machine --- FXS[2851]Fe0/0 --- (IP cloud) --- CCM



voice-port 0/1/1

no battery-reversal

input gain 10

timeouts call-disconnect 1

timeouts wait-release 1

station-id name FAX

station-id number 1111


dial-peer voice 1 pots

destination-pattern 1111

no digit-strip

port 0/1/1


dial-peer voice 2 voip

preference 1

destination-pattern [1-9]...

session target ipv4:x.x.x.x

dtmf-relay h245-alphanumeric

fax-relay ecm disable

fax protocol cisco

no vad

This g323 GW (2851) has the following settings on the CCM: there are two Locations within two Regions. There is a g729 codec between these two regions.

Voice calls are fine but faxes through VoIP not. This router has a FXO port and I can send faxes to this port.

But all (fax and voice calls) work fine w/ g711 between these regions.

Transcode resurses fork fine.

What can be a problem?

How I can configure all voice calls w/ g729 and fax calls w/ g711? There are so many fax numbers through VoIP so I can not match all of these w/ dial-peers.

I have this problem too.
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Overall Rating: 3.5 (2 ratings)
Avner Izhar Mon, 08/06/2007 - 09:19

Where are those destination fax devices connected, are they on devices which can be set to the G711 region?

ipovarenkin Mon, 08/06/2007 - 23:41

Hi Avner,

Thank you for the reply!

All these fax devices are connected through different ways.

Many of these fax machines are on the VG248. This VG registered through SCCP on the CCM.

There are also many devices in the PSTN w/ the following fax flow:

FAX(source) ---> FXS[2851]Fa0/0 >--- (IP cloud) >--- CCM ---> [h323 GW] >--- (PSTN) >--- fax machine(destination)

For the first case I can set the g711 between these regions (VG's region and GW's region) but what I need to do in the second case?:)

This is a Multisite Deployment Model with Centralized Call Processing and I can not change these dial-peers from pointing to CCM to pointing to another h323 GW...

ipovarenkin Tue, 08/07/2007 - 00:44

And another one additional thing, I can plug an analog phone to this FXS port and successfuly call thgough VoIP w/ g729.

So the voice calls from FXS port are ok w/ g729 between regions but fax calls do not.

Is this a router's DSP issue?

It is lools like the DSP can not recognize FAX tones...

botsford-e Tue, 08/07/2007 - 02:24

Question, do you hear the faxes trying to negotiate? If so fax relay does not usually support Error Correcting Mode (ECM). So verify ECM is turned off on both fax machines.

Avner Izhar Tue, 08/07/2007 - 07:45

I Don't think this is a dsp problems, voip and fax trigger different things so one might work when the other don't.

It looks like your 2851 is trying to send faxes using cisco fax relay and the recieving end might not be configured for that. Also, faxing over ip is a challenging issue so one should try to eliminate it as much as possible.

Not sure what your topology looks like, but try to avoid teho of fax calls over the wan, have a separate route pattern for the fax that sends it via the local gw always.

As for further troubleshuting, the following url might help:


Good luck, Avner.

ipovarenkin Tue, 08/07/2007 - 22:48

Hi Avner,

Thanks I saw this docu and trying to troubleshoot this issue but w/ no luck:(

There are a few remote sites in this installation w/ 2851 routers w/ the same HW configuration and all these branch offices fork fine w/ fax machines connected to the VG248. All settings (IOS version, CCM settings, dial-peers) are the same (rechecked many times).

I don't know what do I do...


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