SIP T.38 <-> G711

Unanswered Question
Mar 13th, 2009


We are trying to get a fax server solution working between CUCM and our SIP provider but it seems the fax server only supports T.38 and the provider only supports G711alaw.

Fax Server (t.38) <-SIP TRUNK-> CUCM 6.1.3 <-SIP TRUNK-> Service provider (G711alaw)

Does anyone know of a product that can act as a gateway between the t.38 and G711 RTP paths? I was hoping a CUBE would be do the job but I can't find any information regarding fax, other that CUBE supports t.38.

Also we have a number of ATA's in the cluster, does any one know of any plans for ATA's to work with t.38?



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Nicholas Matthews Fri, 03/13/2009 - 20:26

Hi Karl,

CUBE only supports the same fax codec on both sides.

The solution for something like this is to make it a 'fake CUBE'. This includes looping back a T1 on the same router, and making the router think it's originating and terminating two analog legs.

Topology is something like this:


It's sort of wacky, but it should work.

The ATA's are legacy products, and I wouldn't expect new features to come out for them. The replacements, the VG202 and VG204, have all the capabilities of an IOS gateway like the VG224. They support every method of faxing that an IOS gateway does - you may want to look into these if this is an issue for you.



karlfisher Thu, 03/19/2009 - 11:56

Thank you for the information, we are going to try the setup to see if we can at least use it as a temp. workaround.


Karl Thu, 10/08/2009 - 14:06

did you ever get a permanent solution to this issue of g711 to t.38 transcoding? We are running into the same problem. Did the PRI "trick" work for you?

karlfisher Thu, 10/08/2009 - 14:25


No, we did not implement the PRI trick. It turned out to be easier to change to a service provider that did support T.38 then try to source the equipment.



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