With regard to faxing, I realize you keep pointing out that it used to work and now it doesn't. Nobody here knows why that is the case. Faxing over VoIP is never going to be as reliable as using a traditional POTS line. Sometimes, VoIP providers change the "transit" carriers they use to route calls, and the quality of the connection no longer is sufficient to support faxing. That's just one example.
Drgeoff pointed you to some tips. He specifically emphasized that you need to turn off ECM (error correction mode). This seems counter-intuitive, since error correction sounds like a "good thing". The problem is, it's only a good thing for traditional circuit-switched (POTS) telephony. It identifies failures by the receiving end to receive error-free transmissions. If it detects an error, it tells the sending end to re-transmit. This can create a mess with VoIP, which results in endlessly re-transmitting the same packets over and over until it gives up. Also, as he pointed out, try reducing the data rate on the fax machine. 9600bps is the maximum that is likely to work over VoIP, and 4800bps is more reliable. Both the data rate and ECM mode are settings on your fax machine, not on the OBi.
The one thing you can try to change on the OBi device, using its Expert configuration mode, is to change your CODEC profile to only use the G.711 CODECs, and no others. G.711 is a lossless CODEC that works best with faxing, however, it does require a high-quality internet connection (from your own internet service provider).
What if this doesn't help? Here are two other options:
Get a Google Voice phone number. Google has significantly improved its VoIP network over the years, and it is often more reliable for fax than other providers.
or...
Join the 21st century. Don't use your fax machine. Instead, scan your documents and email them as PDFs, or use a fax service provider that can fax the PDFs for you. Get a fax mailbox from the service provider of your choice, to receive inbound faxes, which are then converted to PDFs that can be downloaded or emailed. I don't know if Anveo offers them, but Callcentric does.