Can every SIP extension be voice/fax (hylafax)

We’ve been testing hylafax/nvfax on a server and it works just fine.
We installed and started playing with avantfax as well but it doesn’t appear to be quite what we’re in need of.

The server will have some 100/200 extensions on it, say 30/40 concurrent calls (estimate).
Rather than have to mess with a separate fax server and accounts, we would prefer to have each extension be able to accept either voice or fax calls but aren’t sure about these capabilities.

We set up an IAX extension with IVR and fax on it. It answers fine on a voice call and it auto detects a fax call and accepts faxes.

We then tried to do the same thing with a SIP extension, we can receive voice calls but fax auto detect doesn’t work. We thought that since our provider is T.38, that faxes are coming in over the SIP trunk, that asterisk would detect it and route that to the IAX modem then a notice to the users mailbox.

Can anyone shed some light on what we’re missing here.