SIP Trunk with MTP and fax passthrough

Unanswered Question
Oct 20th, 2008
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Hello guys, i would like to know if fax passthrough is supported with a SIP Trunk with "MTP is required" checked ?

I had to check the "MTP is required", because of conferences with outside users that did not work and because of PSTN prompt not played correctly to ip phones. As soon as i checked the box, fax transmissions failed.

CUCM 5.1(3) + 2801 gateway

GW configuration for fax passthrough:


voice service voip

modem passthrough nse codec g711alaw



dial-peer voice 50 voip

destination-pattern [45]...

voice-class codec 1

session protocol sipv2

session target ipv4:

dtmf-relay sip-kpml

fax rate disable

no vad


Thanks a lot for your comments.



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irisrios Fri, 10/24/2008 - 06:41
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Software MTP does not support T.38 fax or fax pass-through. With 5.1 CM, you need hardware MTP for your scenario.

jtoque Fri, 10/24/2008 - 06:58
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Hello, thanks a lot for your answer.

I have to try this settings:

"MTP is required" on the Trunk to be able to listen to pstn messages.


Hardware MTP for fax passthrough support.

Do you mean that with CUCM 6.X, software MTP support passthrough and T.38 ?

Thanks again.


jtoque Wed, 11/12/2008 - 09:18
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Hello, i configured hardware MTP on 2801 gateway + hardware transcoder (if needed).

=> Fax transmissions failed, passthrough was not operating correctly.

I did not understand why dsp ressources on 2801 were not allocated when a call was placed through the SIP Trunk with MTP required checked (no need ???, show sccp connections did not show any allocation).

I have placed the hardware ressources at the top of Media Ressource Gpe List and removed software MTP from this MRGLST and reset the trunk.

Any help would be precious !



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