FortiVoice Fallback to audio (G.711) mode on T.38 failure

[ryanw]

Hello,

I have a FortiVoice 200F8 verison 7.2.1 running a VOIP trunk with fax setup. T.38 is enabled on the trunk SIP profile. I took a packet capture of an inbound fax and this is what I found (I am the client). 

Maybe I am misinterpreting the packet capture, but it looks like my 200F8 is trying to use T.38, the sender ignores it, and then the FortiVoice hangs up. What I find interesting is this setting under Fax:

The packet capture shows multiple attempts at T.38, doesn't fallback to G.711, and ends the call. What constitutes T.38 failure for this setting to work? 

[Jean-Philippe_P]

Hello ryanw, 

Thank you for using the Community Forum. I will seek to get you an answer or help. We will reply to this thread with an update as soon as possible. 

Thanks, 

[Jean-Philippe_P]

Hello,

We are still looking for an answer to your question.

We will come back to you ASAP.

Thanks,

[Jean-Philippe_P]

Hello again ryanw,

I found this solution. Can you tell us if it helps, please?

In the context of FortiVoice and T.38 faxing, a T.38 failure typically occurs when the T.38 negotiation between the sending and receiving devices is unsuccessful. This can happen if:

1. T.38 Reinvite Ignored: If the sender does not respond to the T.38 reinvite, the FortiVoice system may interpret this as a failure.
   
   
2. No Acknowledgment: If there is no acknowledgment or "go-ahead" signal from the receiving terminal after the T.38 reinvite, it may be considered a failure.
   
   
3. Timeouts: If the system waits beyond the configured "Wait time for an answer" without receiving a response, it may trigger a fallback or termination.
   
   
4. Configuration Mismatch: Any mismatch in the T.38 settings between the sender and receiver can lead to a failure.

To ensure the fallback to G.711 works, verify that the "Fallback to audio (G.711) mode on T.38 failure" option is enabled in your FortiVoice configuration. This setting should allow the system to switch to G.711 if T.38 negotiation fails. Additionally, ensure that the retry settings are configured correctly to allow multiple attempts before terminating the call.