When loop-start signaling is used, the FXO interface of a router looks like a phone to the switch, a private branch exchange (PBX), public switched telephone network (PSTN), Key-System, to which it connects. The FXO interface closes the loop in order 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, for example, an FXO interface, in order 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 in order 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.
The most common symptoms of this problem are phones that continue to ring when the caller has cleared, or FXO ports that remain busy after the previous call should have been cleared.
Refer to these documents for workarounds and further background information:
Understanding FXO Disconnect Problem
Understanding Foreign Exchange Office (FXO) Voice Interface Cards
Hope this helps.