Why applications like spandsp is not suggested in conjunction with T.38 is because it requires SIP, and unless Zimbra is a PBX, this may not be the way to go. So that is why a fax hardware will be needed such as dialogic fax boards which go over the PSTN. I will suggest to browse the Callweaver source code to see how they implement T.38, perhaps that will help or even be portable to Zimbra;
wiki . callweaver .