it does help. Voice supervision has to be turned on.
On the pstn trunk group, put these options:
Cas Signaling Type = loop start fwd disconnect(6)
Cas Orientation = user(0)
Supervision = answer and disconnect(3)
But this is not a solution. You will always have such a problem, because analog phone lines (unlike E1/T1) do not mostly have disconnect supervision capabilities. You can solve your problem by buying those line supervision boxes, which stay on the way between the PSTN phone line and Quintum. They cost about 30-40 EUR each.