I gave up long ago on the "white whale" of working Fax over IP even with T.38. It's so hit and miss.
the problem is that when you are about to give up entirely, you'll have one or two work - and it keep bringing you back into it - thinking that you might have figured it out....
I have switched all of my customers over to Vitelity vFAX: http://www.vitelity.com/services_vfax/
This service is rock solid, they give you an "ATA" which really intercepts the fax messages, converts them to TIFF and sends them up to a server via HTTPS where they get faxed out on the PSTN. Works quite brilliantly, and saves you from having to retrain your customers on how to use FAX portals from their PC when all they want to do is just use a fax machine like they always did.