FAXing over VoIP networks doesn't work. You can sometimes arrange things so a fairly high percentage of FAXes get through OK. You can occassionally create setups that work 100% of the time. These are rare and unrepeatable setups. You need to use a proper FAX over IP protocol, such as T.38, to achieve consistent reliable FAXing across IP networks.
FAX, and other modem applications, operating over VoIP channels are quirky, and unreliable. This will not get better over time. It will get worse. In general, the more sophisticated the equipment gets in trying to make speech work smoothly, the worse it behaves for modems. In the near term (i.e. until all data applications are native IP applications) store and forward protocols, and protocols tailored to reasonably conveying modem data across an IP channel are the only way to achieve consistent results.
Ron Stevens
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