The general advice for faxing over voip is to lower speeds to 9600 Baud and ensure that only uncompressed codecs are used such as G711. OBi devices do use G711U and G711A codecs, so that side should be ok. You would probably have to go peer to peer, not via a voip provider, to ensure that signals don't get compressed by the voip provider.
I believe that there are some differences between how faxes and straight modems work, but you need someone more expert than me to talk about that! All I know is they both push data down telephone lines.
If your two modems can seize a telephone line and auto-answer when they receive ringing, then the above azrobert config should work. You might need to experiment with the config to eliminate connection delays. Also note, that the original question concerned two OBi devices on the same subnet. If you try this project out, then it would be easiest to set it up on the same subnet first for easy fault finding.
Anyhow, looks like an interesting project. We would all like to hear about it if you give it a try - we might even help