I have seen here that many people say that GV and Obi work "out of the box" for faxing, however in my tests I was unable to reliably connect at any speed higher than 14.4kbps with Obi "out of the box".
I looked around and found the five settings in the Physical Interfaces > Phone 2 settings
1 Disable call waiting (Checked)
2 Use for Fax Only (checked if using dedicated fax only)
3 Fax Detection Medthod (CNG or CED)
4 Detect Phone CED Phase Reversal (Checked)
5 Detect Network CED Phase reversal (Checked)
With the above settings on my GV dedicated Fax line on port 2 of Obi202 I have managed to fax at 33.6kbps far more reliably.
Updating this post to show the additional Fax settings that I've found for my dedicated Fax line
FaxDetectionMethod (CNG or CED)
I just put my Canon Cdx on my Port 2. It worked but wasn't anything to write home about. Tried your tweaks and while I'm not sure exactly what speed it's FAXing at, both the outbound and inbound using Canon's Automated FAX test system were MUCH faster.
I actually surprised myself when I recently sent a couple faxes across the state and was sure they failed due to the very short time it was connected. In the end the faxes were both fine with these settings and very fast from My Dell MFP.