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Modem Relay/Passthrough and Fax Relay/Passthrough

Is it possible to have a combination of both Modem and Fax Relay/Passthrough? using 2620XM's and 3660's with any version of IOS. Is this supported? Can some one point me to some example configs?

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Cisco Employee

Re: Modem Relay/Passthrough and Fax Relay/Passthrough

It depends whether the GW is MGCP or H323.

With H323 you can do modem passthrough and fax relay concurrently, just use the standard config lines for each feature. This is a more difficult question with MGCP right now, FAX should work but I don't believe that modems are supported (yet)

New Member

Re: Modem Relay/Passthrough and Fax Relay/Passthrough

Do you happen to know what version of the IOS that this works with? The problem I have is when both modem passthrough and fax relay are configured on the voip dial-peer. We use connection trunks to emulate T1 across our Gig backbone. The PBX always starts with the last channel (24 or 12) available to establish a phone call. So let’s say that a fax is sent and it uses channel 12. After the fax is complete I can no longer make a voice or fax phone call. Again this is only when modem passthrough and fax relay are both enabled. What happens is a fax will go through just fine but during a debug I see a dsp protocol error. That is why I would like to know what version of the IOS this has been proved in and if there are and configs that show both being used. It is amazing what uses a modem to communicate. (Elevators, alarm systems, credit card machines, etc...) So it’s kind of important that I can get this to work.

Thanks.

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