I had this same problem with faxing over a VoIP line to my own MyFax account. The solution seems to be setting your fax machine on international mode where it sends it a different way. I believe that MyFax can actually switch your account to a different server which will work a little better. Let me know if you guys had success and what you told them, so I can try that for myself.
The ma'ala of MyFax is that for $10 a month you get incoming AND outgoing (I just fax to 7185555555@myfax.com from a registered email address and they send the fax through). With pfax, which I think it only $3 or $4 a month, you only get incoming. I personally need SOME outgoing so I went with MyFax.