Unplug the fax line and call it again, if still busy the machine is not the issue.
If not busy, how long after the printer is plugged back into the line until it is busy again?
If the printer is using the line, listen to it by turning on the line monitor to see what it is doing (Machine Status > Tools > System Settings > Common Service Settings > Audio Tones > Line Monitor Volume and crank it up ;-)
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