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Re: Ask the Expert: Configuring and Troubleshooting Faxing in Ci

Hello Chris,

     Thank you for the feedback and for posting your question. With regards to SIP trunk integration with CUBE SIP providers, my suggestion would be to verify if they support standard RFC based T.38, please see it referenced below. Also since nse based modem passthrough is a Cisco proprietary protocol it won't work with 3rd party vendors such as your SIP carrier. As for fax protocol based pass-through it is using a Cisco pre-standard which in certain scenarios will cause issues with 3rd party vendors and certain signalling protocols. T.38 provides better stability and reliability when integrated with most protocols including your carriers SIP trunk if they support it, otherwise if there is no fax protocol negotiated the fax will simply send the fax tones using a normal voice call which may or may not work depending on the certain circumstance.

     To keep fax configuration simple, maintainable may I suggest using one fax protocol globally on your cluster as a mix of two or more may result in intermittent fax failures. Given the limited options with integrating 3rd party devices T.38 most likely what will be supported by other vendors. Saying the above you need to choose devices and signalling protocols which support fax protocol T.38 on your network.

     Two limitation, firstly SCCP controlled FXS ports do not support protocol based fax protocols such as protocol based fax pass-through and protocol based T.38.  Thankfully though IOS gateways such as the vg224, can be configured with multiple voice protocols. For example you can leave your analogue handsets running SCCP for supplementary services such as hold, transfer etc and since MGCP supports t.38 my suggestion would be to move the FXS ports on the vg224 that are only connected to fax machines on to the MGCP protocol. Secondly MGCP gateways do not support fax protocol based pass-through and so this narrows the option to only fax protocol T.38.

     The configuration would be simply disabling Cisco Fax protocol, and all other fax and modem protocols and only leaving t.38 on the UCM, the vg224, the gateway and the cube. Please find the following sample mgcp and sip fax t38 configuration.

!#### CUBE global fax configuration
voice service voip
fax protocol t38 nse version 0 ls-redundancy 0 hs-redundancy 0 fallback none
no modem passthrough nse codec g711alaw
!### CUBE SIP dial-peer
dial-peer voice x voip
fax-relay ecm disable
fax rate 14400
fax nsf 000000

!#### VG224/gateway mgcp configuration
no ccm-manager fax protocol cisco
no ccm-manager config
mgcp package-capability fxr-package
no mgcp fax t38 inhibit
mgcp fax rate 14400
mgcp fax t38 nsf 000000

     The above sample configuration should take of most issues such as ECM, Speed and NSF. However at times the ECM, Speed and NSF values need to be adjusted on the physical fax machine, this why I recommend to disable ECM and speed to 14400 on the actual fax itself.  To explain the NSF values, they are basically the fax machines manufacturers code, and if both fax machines negotiate the same manufacturers code they will send embedded proprietary T30 signals that are not supported and cause intermittent issues.

     For your reference please review the following documents for commands references and the t.38 standard:

Fax Relay Troubleshooting Guide

Configuring T.38 Fax Relay

SIP Support for Real-time Fax Call Flow Example and  Best Current Practices

Please let me know if you have any further questions.

Thank you



Edson Pineiro



Ask the Expert: Configuring and Troubleshooting Faxing in Cisco

Hi Edson,

thank you very much for your great help! I have changed our VG224 from SCCP to MGCP to do only T.38 and it works perfect in all scenarios (isdn and sip trunks to provider). Because we have no analog phones connected to vg224, wich need supplemental services, we don't need SCCP and MGCP at the vg224. So I have configured CA-controlled MGCP at vg224 and now configuration of new ports is easy as SCCP before, because the dial-peers will be configured automatically from callmanager, when the new pots ports is configured in callmanager. So there is no need to work at vg224-console in normal business.

Regards, Chris

PS: Im sorry, but I want to rate your post with 5 stars but I had clicked wrong to only 3 and I cannot revote... :-( sooorry.


Ask the Expert: Configuring and Troubleshooting Faxing in Cisco

Hello Christian,

     I really appreciate your feedback, it's always good to know the outcome of my suggestions. I'm glad the fax is working for you, in all scenarios.

     The most important recommendation for anyone doing fax over IP, is to choose one fax protocol and design the complete cluster using only the selected fax protocol ensuring all end devices support it. An optional addition to the above point would be to disable SG3 and ECM, as this fixes most of those intermittent fax failures.

     If anyone has any further questions, please do not hesitate to post me a note on this forum.

Thank you



Edson Pineiro



Ask the Expert: Configuring and Troubleshooting Faxing in Cisco

In regards to the fax onramp tcl script, are there capacity/volume limitations?  I realize that it'll vary between isr platforms, but even general details or real world usage examples will be helpful.  Also, what are some good troubleshooting commands?  Is there a way to see the number of current sessions -- maybe active and history?  Just viewing the call active/history won't single it out as we have other faxing traversing our voice gateways.  Any info is welcome.




Ask the Expert: Configuring and Troubleshooting Faxing in Cisco

Hi Edson,

I'm facing an issue with our new ATA 187. We have recentely deployed 2 new ATA 187, when I send fax internal between 2 fax machines that are conncted via ATA it works!!!


when I try


or vise versa it doesnt work.

And the ATA 187 is using T38 Fax Relay

And the Sip Gateway is configured:

voice service voip

fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback none

allow-connections sip to sip

dial-peer voice 1123 voip

translation-profile outgoing Calls

destination-pattern 9.T

session protocol sipv2

session target ipv4:X.X..XX

incoming called-number .

voice-class codec 1

dtmf-relay rtp-nte

no vad

Is their any fix for this.


Its my first to configure fax

Its my first to configure fax in any voip network even working since 2 years. Consider two scenarios. I have

Sip trunk from ITSP connected with 2921 Router, cucm version 8.x. Internal extension are replaced in DIDs using rules on the GW.

Scenario 1:



Scenario 2:



My fax machine will be directly connecting with FXS card. Please help me with step by step config as i never cofigured fax before


Thanks in Advane


Hi Edson,We are using Third

Hi Edson,

We are using Third Party FAX server (FAX CORE) in our setup.


CUCM 9.0

Cisco Voice Gateway (CUBE) - CISCO 2821

IOS - c2800nm-spservicesk9-mz.151-4.M6.bin

IP FAX Core Server (Third Party FAX Server)

Inbound Fax Call Flow


Outbound Fax Call Flow

IP FAX server—SIP Trunk—CUCM—H.323 Gateway Trunk—CUBE(VG)—PSTN

We are using G711ulaw for both inbound and outbound fax calls.


Both Inbound and Outbound fax calls are not working.

Incoming Fax call are landing to fax server and call got disconnected with following error.

SIP/2.0 488 Not Acceptable Media

Wire shark Flow Diagram from FAX server: - CUCM - FAX Server - Gateway(CUBE)

Wire shark Flow Diagram for Inbound FAX call

In Outbound FAX calls, calls were answered by destination fax machine but fax message is not receiving by fax machine.

I have attached the wire shark logs which is taken from FAX server (logs only for Inbound FAX call).

Please let me know if you required any other information from our end.

Kindly help me to solve this issue.


Did you ever resolve this

Did you ever resolve this issue? I have a very similar issue and am unable to resolve it. Any information you have on this would be great.


Thank you!


This issue solved for me.Our

This issue solved for me.

Our service provider do not support t38 protocol for FAX calls.

I am using 'pass-through g711ulaw' instead of t38.

Failure Call logs which was generating when t38 protocol not support by Service Provider.

Inbound FAX from PSTN to CUCM:

Call details:

Calling Number: 0112790986

Called Number: 2188787


CUBE sends a re-invite with T.38 Capabilities to the ITSP


INVITE sip:112790986@;user=phone SIP/2.0

Via: SIP/2.0/UDP;branch=z9hG4bKE5FC2213

Remote-Party-ID: <sip:8787@>;party=calling;screen=no;privacy=off

From: <sip:2188787@;user=phone>;tag=19ABCCA4-B5

To: <sip:112790986@;user=phone>;tag=sbc0804ectp2bs4-CC-31

Date: Wed, 06 Aug 2014 11:26:30 GMT

Call-ID: isbc7skbusdappc2cbua7o4aspesccbc4hdh@SoftX3000

Route: <sip:;transport=udp;lr>

Supported: 100rel,timer,resource-priority,replaces,sdp-anat

Min-SE:  1800

Cisco-Guid: 1459576986-0479400420-2241498593-3224978540

User-Agent: Cisco-SIPGateway/IOS-12.x


CSeq: 101 INVITE

Max-Forwards: 70

Timestamp: 1407324390

Contact: <sip:2188787@>

Expires: 180

Allow-Events: telephone-event

Content-Type: application/sdp

Content-Length: 400


o=CiscoSystemsSIP-GW-UserAgent 5215 7761 IN IP4

s=SIP Call

c=IN IP4

t=0 0

m=image 17352 udptl t38

c=IN IP4










ITSP respond with Trying:


SIP/2.0 100 Trying

Via: SIP/2.0/UDP;branch=z9hG4bKE5FC2213

Call-ID: isbc7skbusdappc2cbua7o4aspesccbc4hdh@SoftX3000

From: <sip:2188787@;user=phone>;tag=19ABCCA4-B5

To: <sip:112790986@;user=phone>;tag=sbc0804ectp2bs4-CC-31

CSeq: 101 INVITE

Content-Length: 0


And then “Media Unacceptable”  from Service Provider End



SIP/2.0 488 Not Acceptable Here

Via: SIP/2.0/UDP;branch=z9hG4bKE5FC2213

Call-ID: isbc7skbusdappc2cbua7o4aspesccbc4hdh@SoftX3000

From: <sip:2188787@;user=phone>;tag=19ABCCA4-B5

To: <sip:112790986@;user=phone>;tag=sbc0804ectp2bs4-CC-31

CSeq: 101 INVITE

Content-Length: 0


Hello Edson Pineiro       Now

Hello Edson Pineiro

       Now I have problem incoming fax


CUCM 10.0

Cisco Voice Gateway (CUBE) - CISCO 2900

Imagicle cisco for UC (Stone fax)


Inbound fax

PSTN (ISP)—SIP—CUBE—SIP Trunk—CUCM—SIP Trunk—IP FAX server(stone fax)


Outbound Fax Call Flow

IP FAX server(Stone Fax)—SIP Trunk—CUCM——CUBE(VG)—PSTN(ISP)

Now we are using G711alaw for both inbound and outbound fax calls.



Inbound fax can receive some number such as my office , my home ,my home friends and my office friends

for outbound it working


Wireshark log and flow diagram -Voice Gateway (CUBE) - CUCM - Fax server (Stone fax)



I have attach log from wireshark.Could you please suggest for any my mistake configuration? and Please let me know if you required any other information


Please Kindly help me to solve this issue.











Hi Edson,We are getting

Hi Edson,

We are getting transmission error when we sent faxes to few numbers (outbound calling), mostly international numbers. The Right FAX vendor is claiming that the issue is caused due to line quality issues. Our setup is as below


RightFAX - H323 - Cisco VG - PRI - Service provider (Not touching the CUCM)


We are using T38 for fax communication. How can I verify the line quality is affected in VG or not. I have attached the backup from the VG


Please find the debug

Please find the debug attached, in the debug I can see 

timestamp=1943793553 FR_BAD_CRC_LS_DATA 0x0 bytes 
timestamp=1943793553 fr-msg-tx  bad crc, 0 bytes

Any idea how the the bad CRC would hav been formed, when i checked under the controller there is no input/CRC errors


Hi EdsonHope you doing wellwe

Hi Edson

Hope you doing well

we have configured and installed cisco ATA186 in CUCM 9.1 & VGW H.232 with SIP line from ISP

we are facing issue with incoming fax, incoming fax missed most of the times and showing missed calls notification on fax machine without any ring for incoming fax...fax machine already set to auto answer


Please advice on the above issue urgently

Thanks in advance and waiting for your kind response

Warm Regards:

Nabeel Asghar



Re: Ask the Expert: Configuring and Troubleshooting Faxing in Cisco IP Voice Networks

Hi Edson

We have cucm setup like this:

SME cluster with
 Cisco 2951 MGCP gateways with ISDN30 connections.

connectied via SIP trunk to

CUCM cluster with
 10+ 2901 MGCP gateways for local backup ISDN30 connections.
 50+ VG202 SCCP gateways for analog devices, mainly faxes.
 30+ VG224 SCCP gateways for analog devices, faxes on some ports.
 20+ VG310 SCCP gateways for analog devices, mainly faxes.

At the moment most of the devices is with default configuration for faxing.

Right now is not very predictable if faxing works or not, and many cases faxes needs to dial more than once to get a fax through the network.

We are looking into doing complete redesign of the fax configurations deployed and my question is very simple.

What is the 2017 recommendation for doing this?

What protocols should be used for faxing?

What protocols should be used for gateways? sccp/mgcp/sip?
(keep in mind that changing protocols is not an easy task based on the number of endpoints deployed)

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