The one thing everyone asks about in VoIP is FAX. Yeah, yeah. T.38. G.711. Blah. Blah.
If the entire VoIP network runs on a single platform built with Brooktrout Session Border Controllers, then fax will work.
Where does Fax over IP work? well, a short outbound fax will usually work with a fax port on a Edgewater device. But anyone that using a lot of fax should just stay with TDM. The other option is to move to a dedicated fax over IP provider. That's what CallWave just did:
CallWave Sells Virtual Fax Subscriber Base to J2 Global for $12 Million [TMCnet] J2 owns the original efax service.
There are other providers like Premiere Global Fax 2 Mail service that can be white labeled for you. (I rep for Premiere).
Why doesn't fax work well? Compression and Conversion. When you compress a fax packet, it destroys the image file. When you do conversion on the fax packet - like from G.711 to G.729 or analog to digital - it degrades. More than 15% degradation renders the fax unreadable. That is the problem. It has to have end-to-end quality of service. Most VoIP networks are not designed that way. More like duct taped pieces together.