I had a same question:

http://thread.gmane.org/gmane.comp.telephony.freeswitch.user/86263

Sorry google lead me only to this link...

On Dec 7, 2017 18:06, "Callum Guy" <callum.guy@x-on.co.uk> wrote:

Hi All,

I'll try and keep this simple. 

We are experiencing difficulty transmitting fax with a certain UK provider (BT) who have suggested the fix for the fault is that we "should fall back to using G711 pass through in order to negotiate/ send fax"

The call initially connects with G711 (and the media is proxied). 

Fax is detected by the originator which issues a RE-INVITE request to use T38. 

This is rejected by the gateway with a 488 at which point we would expect the call to fall back to G711 however FreeSWITCH is reporting that this is not possible due to a media bug:

"Session is connected to a media bug. Re-Negotiation implicitly disabled."

My understanding was that a media bug would be required for the initial fax tone detection which subsequently triggers the T38 session start? 

Could anyone confirm whether we need to be looking into closing off the media bug manually or if there is an alternative route to resolving this issue?

We are running FreeSWITCH (Version 1.4.26  64bit). 

Calls are dispatched with setting "t38_passthru=true".

Any help would be very much welcomed.

Best Regards,

Callum

--

Callum Guy

Head of Information Security

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