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New Member

t.38 with exchange

i have a 323 gw with cucm 7.x and exchange 2007.

Is anyone out there doing faxing with this sype of configuration and if so would you share gw/cucm configs?



Cisco Employee

Re: t.38 with exchange

Hi Paul,

On the CallManager side, CUCM 7.0 introduced SIP T.38 interoperability with Microsoft Exchange.

Exchange switches a call from audio to T.38 fax by sending SDP with two "m=" lines, one to terminate audio and one to enable fax session. Previous versions of UCM do not use this method, and interprets it as an audio request to terminate the call so fax calls fail.

In UCM 7.0, the implementation of T.38 interoperability with Microsoft Exchange enables the system to switch a call from audio to T.38 fax.

Cisco Unified Communications Manager Administration allows you to configure a SIP Profile that supports T.38 fax communication. This profile applies to SIP trunks only, not phones that are running SIP or endpoints.

The Outgoing T.38 INVITE Include Audio mline parameter allows the system to accept a signal from Microsoft Exchange that causes it to switch the call from audio to T.38 fax. To use this feature, you must also configure a SIP trunk with this SIP profile.

To access the Outgoing T.38 INVITE Include Audio mline parameter, choose Device > Device Settings > SIP Profile for the SIP trunk to Exchange. Select the checkbox for "Audio line for T.38 Fax Calls"

See also:

Hope this helps.



New Member

Re: t.38 with exchange

Thanks for the info - missed that one for sure.

so on router i have the following

voice service voip

fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco

do i need a specific dial peer to fire this through as sip to the cucm?

guess i'm unclear on the logistics of it such that do i give out my did as fax as well or do i assign another number to it that goes directly to exchange?

apologize for the total lack of insight on this one.

Re: t.38 with exchange

You will need a separate number for your fax. That is the best way to route these calls. Now as for routing it really depends on how you UM setup. If it is a SIP trunk directly out of call manager then you can send the calls to CM has h323 and out CM as a SIP Trunk. Keep in mind the SIP parameters will need to match correctly. If you have a gateway, then you will then send the call directly to UM via sip.

I hope this helps, Please rate post.

New Member

Re: t.38 with exchange

seem to be getting closer but would like to find someone that has this working.

the call comes in but not recognizing the CNG tones for some reason....

any help would be appreciated.


New Member

Re: t.38 with exchange

I realize this is a little bit late, but it may help future searchers:

In order to get exchange to recognize the CNG tones, you have to modify the globcfg.xml file in the Bin directory of your exchange installation path ("program files" directory unless you installed it elsewhere). In the file there is a directive "EnableInbandFaxDetection", set that to true and restart the unified messaging service. With this option, exchange will issue a re-invite to the cucm to switch the call to fax. If memory serves correctly, it is only available on exchange 2007 sp1 and later.


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