When loop-start signaling is used, a router's FXO interface looks like a phone to the switch (private branch exchange (PBX), public switched telephone network (PSTN), Key-System) it connects to. The FXO interface closes the loop to indicate off-hook. The switch always provides a battery so there is no disconnect supervision from the switch side. Since a switch expects a phone user (example of an FXO interface) 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 the battery to indicate on-hook). Because of this, there is no guarantee that a near-end or far-end FXO port disconnects the call once either end of the call hangs-up.
For further information click this link.
http://www.ciscosystems.com/en/US/tech/tk652/tk653/technologies_tech_note09186a00800ae2d1.shtml
