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CUCM 7.1.5 FAX server abort call if CED tone is received after 20 secs


I'm working on this problem, where outgoing faxes fails if Cisco GW receives the CED tone from the Terminating fax machine after 20 secs. Here is the topology

FAX server --- H323 ---> CUCM 7.x ----> HW MTP ---> H.323 GW --> ISDN PSTN --> Fax machines

CUCM allocates a HW MTP for the H.323 GW since it uses FS for another devices. So far some faxes works and others dont' but what is common to all failed faxes is that the CED tone is being recieved after 20 secs. Here is the isdn debug:

008690: .Jan 17 21:06:59.807: ISDN Se0/0/0:23 Q931: TX -> SETUP pd = 8  callref = 0x0334

    Bearer Capability i = 0x8090A2

        Standard = CCITT

        Transfer Capability = Speech 

        Transfer Mode = Circuit

        Transfer Rate = 64 kbit/s

    Channel ID i = 0xA98397

        Exclusive, Channel 23

    Calling Party Number i = 0x0081, N/A

        Plan:Unknown, Type:Unknown

    Called Party Number i = 0xA1, '16137328809'

        Plan:ISDN, Type:National

008691: .Jan 17 21:06:59.939: ISDN Se0/0/0:23 Q931: RX <- CALL_PROC pd = 8  callref = 0x8334

    Channel ID i = 0xA98397

        Exclusive, Channel 23

008692: .Jan 17 21:07:01.187: ISDN Se0/0/0:23 Q931: RX <- ALERTING pd = 8  callref = 0x8334

    Progress Ind i = 0x8088 - In-band info or appropriate now available

008693: .Jan 17 21:07:13.247: ISDN Se0/0/0:23 Q931: TX -> DISCONNECT pd = 8  callref = 0x0334

    Cause i = 0x80AF - Resource unavailable, unspecified

008694: .Jan 17 21:07:13.387: ISDN Se0/0/0:23 Q931: RX <- RELEASE pd = 8  callref = 0x8334

008695: .Jan 17 21:07:13.387: ISDN Se0/0/0:23 Q931: TX -> RELEASE_COMP pd = 8  callref = 0x0334

NOTE: I received the H225.0 INCOMING PDU  releaseComplete : from CUCM that's why the GW sends the Disconnect. Clearly CUCM doesn't know that this is a fax call. For CUCM is just a call that keeps in the alerting state.

What I haven't figure out if is the CUCM h225 stack process or the SCCP process (for the IOS MTP) that is sends the release, it looks like there is a timer either in H323 or the SCCP or could be both, where the call cannot be too much time in the ALERTING state before it triggers the disconnect.

So any of you guys/girls know what is the timer in H.323/CUCM or in the IOS MTP/Xcode

Or has faced the same issue?

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