Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Problem sending faxes through VoIP

Hello every one!

I have a problem sending faxes through VoIP

This is what we have:

PBX with VoIP Card --- Cisco --- E1 --- (Internet VPN) --- T1--- Cisco --- PBX

The cisco router has activate the QoS, to priority the communication saving 512 kbps of bandwidth, for the matching VoIP ports.

The voice and faxes work fine, but when the bandwidth is over use, the voice is not affected but the faxes can not communicate even when the QoS is saving bandwidth for them. I been trying with traffic-shape, fax passthrough , fax relay and other options, but no one fix the problem.

I figure out that the cisco router is working only as data gateway not as voice gateway, because the VoIP card in the PBX is making job, so may I?m look to solve the problem in the wrong place.

The VoIP card is configured by a provider, but I try changing the communication options between G.729, G723 and G.711, when use G.711 the voice is better, but the same happen with the faxes.

The question is:

Do I have to correct the fax problem in the VoIP card configuration or is possible to fixed in the cisco Router?

If it can be in the Cisco router, I will appreciate any suggestion.

Thank you very much, regards.


Re: Problem sending faxes through VoIP

The issue is due to slips on the 2811's T1 Controller. Whilethese won't bother voice calls they will prevent modem and fax calls from completing.

Try adding the following

network-clock-select 1 t1 0/0/0

controller T1 0/0/0

clock source line primary

Clear the counters and see if you see the slips continue to increment on the 'showcontrollers t1'

Add the following to both the VG224 and the 2811

no mgcp fax-relay sg3-to-g3

mgcp fax t38 inhibit

no mgcp fax t38 ecm

no ccm-manager fax protocol cisco

New Member

Re: Problem sending faxes through VoIP

Thank you so much,

I been trying to configure you recomendation, but I can?t find the network-clock-select command, I looked it into the general and interface configuration, but it?s not there, I don?t understand why because it support that this command must to be in the Cisco OS version that I?m using.

Do you know if I have to enable some module or update the OS in order to use that command?

Here is the system information.

Cisco Internetwork Operating System Software

IOS (tm) C2600 Software (C2600-IK8O3S-M), Version 12.2(11)T, RELEASE SOFTWARE (fc1)

TAC Support:

Copyright (c) 1986-2002 by cisco Systems, Inc.

Compiled Thu 01-Aug-02 12:47 by ccai

Image text-base: 0x8000809C, data-base: 0x818188F4

ROM: System Bootstrap, Version 11.3(2)XA4, RELEASE SOFTWARE (fc1)

router2600_2 uptime is 4 weeks, 2 days, 23 hours, 23 minutes

System returned to ROM by power-on

System restarted at 18:49:47 PDT Fri Aug 25 2006

System image file is "flash:c2600-ik8o3s-mz.122-11.T.bin"

cisco 2610 (MPC860) processor (revision 0x300) with 61440K/4096K bytes of memory.

Processor board ID JAD05490ZBG (3906121021)

M860 processor: part number 0, mask 49

Bridging software.

X.25 software, Version 3.0.0.

1 Ethernet/IEEE 802.3 interface(s)

1 Serial network interface(s)

32K bytes of non-volatile configuration memory.

16384K bytes of processor board System flash (Read/Write)

Configuration register is 0x2102

Best regards,

New Member

Re: Problem sending faxes through VoIP


You can try also configuring the following commands under the dial-peer voip you are using for fax calls:

fax-relay ecm disable

fax rate 7200

fax nsf 000000

Some faxes don't work fine when error correction mode is enable. hope could be helpful.


CreatePlease login to create content