Put a DSL filter on your fax machine. Yes, a plain ol' filter used for the same DSL that Bell sells. They can be purchased for about $5, or you can get them for free if you have a Sympatico DSL High Speed account. In other words, place a DSL filter on the Line 1 port of the gateway, and split it from there as needed.

Note that faxing is considered a best effort service over IP. (IP itself is considered best effort). With the nature of the underlying protocols and codec’s supported by most major VoIP providers, it's currently very susceptible to packet loss.

T.30 (which is the PSTN faxing standard) is very sensitive to errors and contains a lot of error correction mechanisms. With RTP using UDP as a transport packetloss is considered a behavior of the protocol and the Internet. Running at a codec like G.711u allows for a full 64KBps channel, but it's blind to any of the error corrections provisioned by T.30. That is to say, IP is blind to those error mechanisms, not either end of the PSTN. This is why longer faxes tend to fail over congested links. (Which can be argued as all links)

An ITU standard called T.38 allows VoIP carriers to transport this T.30 PSTN signaling transparently across an IP transport while adhering to error corrections provisioned by T.30. Primus is currently in the process of testing this. There is no ETA, but it is in the works.