I've OBi202 for a few months. I've been testing it with fax using a Canon multi-function inkjet (has an IP phone line setting) and Windows Fax. Testing used Verizon landline, Google Voice and Callcentric. ISP is Comcast cable and with
voipqualitytest.com I consistently get green lights across the board except for "consistency of service" where I score a yellow light. My experience with voice calls GV and CC quality is excellent; on par with landline. So YMMV with fax compared to me, the OBi hardware is just one link in the chain.
Here are my observations:
1) If fax using the phone line is critical, do NOT use VOIP. If you read the fine print NONE of the VOIP providers guarantees or even recommends fax over their service (including CC, despite supporting T.38).
2) CC supports T.38, GV does not.
3) Short faxes (1-2 pages): I was able to send faxes successfully using both GV or CC at speeds up to 33.6K baud.
3) For longer faxes (20 pages): GV could handle long faxes if I reduced to 14.4 baud. CC would fail at about 7 pages at 14.4 baud (but probably not due to any inherent quality problem - see below).
4) There is an undocumented "feature" of CC where they disconnect the line if there is more than 5 minutes of silence from one party on the call. There is no way to disable this. This is a problem with long faxes when the sending fax transmits data for over 5 minutes and CC terminates the call because it thinks the recipient caller has hung up. Very frustrating, but other than splitting a long fax up, no other way.
5) I was able to use error correction without problems.