According to David Hiers (moderator of the VoiceOps list), "At SIPNOC I found out that SIP Forum has a pretty big project going to get FOIP/T.38 fixed once and for all. [See here]." Fax over IP like HD Voice needs travel along a compatible network from end-to-end to work. That's why HD Voice works great for internal calls but breaks when you call your family on a cell phone or a landline. (Off-net). Fax over IP breaks when there are analog-to-digital conversions that break the fax image. T.38 doesn't work effectively and ubiquiously because how many end-to-end T.38 networks are there? Most VoIP Providers route by least cost routing, not static routes for Fax or HD or whatever. Maybe the SIP Forum can fix this. It won't be soon though, because it is a money issue in many cases. In other cases, it's just that there aren't any compatible carriers in the region.
You would think with all the VoIP exchanges and Enum services that by now, at the least, all VoIP to VoIP calls would be T.38 and possibly HD Voice, but I am reading that HD is more complicated.