Had an issue with an IP fax server (RelayFax with a Dialogic SR140 connecting via SIP trunk to an Adtran SBC) where inbound faxes were working fine and using T38 but outbound faxes were never negotiating T.38 and falling back to G.711 passthrough. As a result we were having some unreliable outbound faxes.
In the Brooktrout Configuration Tool under the T.38 Parameters tab we had to change the delay time for media renegotiation. The “Media Renegotiate Delay Outbound, msec” was set to “-1” and I changed this to 5000 msecs. See screenshot: