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33.6 modem connecting though VG224 hangs up

Adam Van Slyke
Level 1
Level 1

I'm looking for some help in trying to understand and resolve this problem:

One of our contractors uses a US Robotics 33.6 modem from his home office to connect to a modem on our site. He get's the handshake signal, but when he is prompted to enter his credentials, he looses signal.

The generic pathway is

Home Office Modem [US Robotics 33.6] ---> VG224 [Ver. 12.4(24)T] and CUCM Ver 7.1.5 ---> end device modem

On our VG:

  voice service voip

    fax protocol pass-through g711ulaw

    modem passthrough  nse codec g711ulaw

I've read through other troubleshooting blogs on this that it could be because t.38 is disabled; I have since enabled the t.38 fax relay on the VG and within CUCM.

Has anyone else ran into the same issue?



5 Replies 5

Adam Van Slyke
Level 1
Level 1

Additionally,  on our CM Gateway I've enabled:

mgcp fax t38 ecm

just for clarification.

So there is another analog GW on the CUCM side correct?

both GWs have to understand the nse and they both have to go into modem relay mode:


voice service voip

     modem passthrough  nse codec g711ulaw

configured on the other GW?  Sniff between the GWs and make sure the nse(you will see different payload type in the RTP stream) gets responded to - also do a sh call act voice - make sure both GWs go to g711 during the modem call.    If there is any h323 involved make sure you are not hitting the default dial-peer.

Pathway is


I have found this link

Which is helpful but only shows Telco-PRI-GW-CUCM-MGCP-VG224_modem instead of SCCP between Call Manager and VG.

Any help would be appreciated.


We're using IOS 12.4(24)T3, and I have seen on Cisco that T2 had issues with the gateway not being able to seperate modem and fax calls. What I can't find is if this issue was for sure fixed in version T3...?  Any insight on that?