FXO disconnect supervision can be problematical.
When loop-start signaling is used, a router’s FXO interface looks like a
phone to the switch (PBX, PSTN, Key-System) it is
connecting to. The FXO interface will close the loop to indicate off-hook.
The switch always provides battery, so there is no
disconnect supervision from the switch side. Since a switch would expect a
phone user (which an FXO interface looks like) to
hang up the phone when the call is terminated (on either side), it also
expects the FXO port on the router to hang up. This
"human intervention" is not built into the router. The FXO port expects the
switch to tell it when to hang up (or remove battery
to indicate on-hook). Because of this, we cannot guarantee that a near end
or far end FXO port will disconnect the call once
either end of the call has hung up.
Check out this link for possible solutions:
http://www.cisco.com/warp/public/788/signalling/fxo_disconnect.html
