We got a T.38 setup working with T.38 capable ATA. That works well, but it's not a scalable solution for large volume faxing.
Unfortunately all our attempts getting a T.38 soft fax were not all that great. We tried using t38modem with Hylafax, it compiles but doesn't work at all or it's flaky. Callweaver's T.38 gateway seems to work but not with Hylafax so there is no queue and hence it's fairly useless for large volume faxing. Ironically enough so far our best results are with IAXmodem and Hylafax and using G711/ulaw and not T.38!
If someone finds a T.38 solution that works with Hylafax reliably, please post a reply