Unfortunately, faxing over VoIP will never be fully reliable so that's why we don't officially support it.

In ideal situations with a clean connection, it'll work, but since VoIP was not designed for faxing and we can't guarantee it won't work, we just don't support it officially.

From TOS:

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Service Limitations

Due to the nature of all VoIP services, VOIPo service is not designed to be used for data connections with modems, home security systems, fax machines and related devices. It is optimized for voice only and you may experience issues and inconsistencies when using VOIPo service in non-voice scenarios.
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Sorry if there was confusion about this, but if we can't guarantee something will work, we just don't support it because then customers come back to us expecting it to be fixed anytime they have an issue with it.