Faxing over VoIP is not very reliable, no matter which settings you tweak. Two settings you can change on your fax machine, not on your OBi, (in addition to the settings Callcentric recommended) are to limit the fax speed to 9600bps, and to turn OFF fax error correction. It seems contradictory, but fax error correction is terribly crude: when an error is detected, the sending fax machine has to re-transmit a large chunk of the data, and if the error rate is consistently above the threshold, it will keep flailing, trying to resend the page. So, it is not "correcting" any errors via extra data bits being sent for that purpose; it is simply re-sending the same stuff.
If this is something you need to do for business on a frequent basis, and it can impact your business if it fails, I recommend using a commercial fax gateway service provider. In that scenario, you typically email the document to the service provider as a JPEG or PDF, and their high-performance fax servers send the fax for you over suitable telephone service.