We got Hylafax running a few releases ago, purely as an alpha test. Almost all of our customers use an Exchange server equivalent that just works so we didn't really put any more resource in.
If you really want to run Hyla then I would suggest you run it on a separate machine and push DDI FAX at it from SARK (spa-2102 works well for this and its cheaper than a Digium board) or, if you have analogue fax, just bypass SARK altogether. Asterisk takes a short lifetime to detect fax tones so a shared phone/fax line does not work well in practice.
Best
S