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T.38 FAX Detection

Paolo Bratti


I have a problem with the recognition of T.38 faxes; my customer has migrated its telephony provider to T.38 and the fax Onramp does not work.

I think there is a problem in the script recognition, the fax detection. Fax detection script is updated to app_fax_detect.

Configuration is the following:


Service fax_detect flash: / / app_fax_detect.

dial-peer voice 3 pots

Service fax_detect

incoming called-number 123456789


port 0/0/0: 15


dial-peer voice 5 mmoip

service fax_on_vfc_onramp_app out-bound

destination-pattern 9989

information-type fax

session target mailto:

Where is the error?

Thanks to all!

1 Reply 1

Frequent Contributor
Frequent Contributor

1. An incoming call is initially established as a voice call.

2. The gateways advertise capabilities in an SDP exchange during connection establishment.

3. If both gateways do not support T.38, fax pass-through is used for fax transmission. If both gateways do support T.38, they attempt to switch to T.38 upon fax tone detection. The existing audio channel is used for T.38 fax relay, and the existing connection port is reused to minimize delay. If failure occurs at some point during the switch to T.38, the call reverts to the original settings that it had as a voice call. If this failure occurs, a fallback to fax pass-through is not supported.

4. Upon completion of the fax image transfer, the connection remains established and reverts to a voice call using the previously designated codec, unless the CA instructs the gateway to do otherwise. If the CA has been configured to control fax relay, the CA instructs the gateway on processing the call with the fx: extension of the local connection option (LCO).

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