Unless the SPA122 hardware is broken, the on/off hook status is not driven by SPA122 but by connected device.
On-hook is signaled by low impedance of connected phone, off-hook by high impedance.
SPA122 didn't take own decisions, it follow impedance of phone connected.
If seems that your fax is not hanging up the line by self unless POTS line has lost the power. Check J420W documentation - you need to configure it to hangup the line by self after fax has been sent.
