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New Member

T38 Fax Issue

I have a Sip connection between a Cisco 2811 and a AudioCodes (Carrius) system. Call can be completed in both directions without any issues. Our issue is faxing. A fax will complete fine if the recieving fax is not set for "auto receive". From what I can tell the RTP stream is established and then the T38 negotiation occurs. When the fax is set for auto this does not occur. Has anyone experienced this issue?....Is there a way to "delay" the T38 setup to allow for the g711ulaw call to be established first...? Any help would be appreciated..




Re: T38 Fax Issue

Make sure your gateway has this configuration:

voice call send-alert

voice rtp send-recv


voice service voip

allow-connections h323 to h323



no telephony-service ccm-compatible

ip circuit max-calls 100

ip circuit default only

no ras brq

h225 timeout tcp call-idle value 0

h225 h245-address on-connect

h225 connect-passthru

call start slow

session transport tcp calls-per-connection 1

h245 tunnel disable

h245 caps mode restricted

modem passthrough nse codec g711ulaw

After this, you need to delete your H.323 gateway configuration from Cisco CallManager and configure this as a Non-Gatekeeper controlled InterCluster Trunk.

When this is done, the Cisco CallManager seems to respond with the H.245 TCS right away and the call is established.

Note: In 4.1.3, you have the option to configure Cisco CallManager to send the TCS right away and not wait to receive it but there is no check box for this in Cisco CallManager 3.3.3. This is why it works with the Trunk, whose default behavior is to send the TCS right away.

Refer to T.38 Fax Relay for Voice over IP H.323 for more information.

New Member

Re: T38 Fax Issue

Thanks for the reply...we are not using h.323 we are using SIP and there is no CallManager involved...this is simply a cisco SIP gw to a Carrius SIP gw.