[cisco-voip] SIP modem passthrough failure

[Eric Pedersen]

A few weeks ago we converted a 2911 PRI gateway from MGCP to SIP. We have an internal system that occasionally makes outbound modem calls, and they are now failing. The modem is connected to a VG224 still running MGCP.

I have the gateways configured for T.38 fax relay and for modem passthrough (not pass-through!) at both the dial-peer and global level with:

fax protocol t38 version 0 ls-redundancy 5 hs-redundancy 2 fallback none
modem passthrough nse codec g711ulaw

But the 2911 does not initiate the NSE switchover to modem passthrough. The Fax/Modem/Text book says that the SIP gateway needs to see NSE support in the SIP INVITE from CUCM, and that's not there so I assume this is why the gateway doesn't do the NSE switchover. However, the SRND says that MGCP combined with SIP gateways are supported for modem passthrough.

Before I open a TAC case, I figure I'd check here to see if anyone has had a similar experience and found a way to make this work. Unlike MGCP, I couldn't see any commands to force a SIP gateway to do modem passthrough even though the signaling doesn't indicate NSE support.

We're using CUCM 8.6(2a)SU1 and IOS 15.1(4)M4

Thanks,
Eric

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