Hello An and Austin:From telephony 101. There are two king of phone services provided by the phone company that you could use. One is called “loop start”.It asumes that you will be conecting the line to a phone and a human being will be using.You pick up the phone, closing the “loop” to the central office. The C.O. detects that and signal back that is ready to accept digits by sending you the “dial tone”. You hear dial tone, start dialing digits and when you finish dialing listen for a busy/recording or ringback tone indicating that is ringing on the other end. Once you finish the call you hang up, releasing the circuit.
The line used is “dumb”.You, the operator are providing the control of the operation.There is no signal sent back to the originating phone that indicates that the other end still on the line or hanged up, except those audio tones intended for the user.
On the receiving end it goes like this: The line is idle or “on hook”.When a call came in the C.O. aplies “ringing voltage”.That is a sequence of around 110 volts of low frequency (20 t0 30 cycles a second)voltage. On the old days that was used to move a mechanical ringer on you phone.Today ,that voltage is detected by your router and after so many cycles ,the router decides to “answer” or go “off hook”.You enter the digits or place your call on VOIP and the originating person decided to finish the call.Now the trouble starts. There is nothing coming from the phone company to your router that tells your cisco that is time to disconnect and go back to idle.The only “clue” is the lack of a conversation on the line, maybe a busy dial tone. Your line became “hung”.
Posible solutions: I do not know anything about the Cisco router, but if the router could take “ground start” phone lines that will fix you trouble for good.
Ground start phone lines are a little bit more expensive than normal phone lines, but they provide a signal that indicates to the equipment at the end of the line if the remote caller still there or not. The soonest that the remote caller hungs up, a signal will travel trought the phone company network, up to your router telling Cisco to go back idle and get ready for another call,releasing the VOIP channel as well.
The second option will be if the Cisco have a silence detection option. If the line goes quiet for more than so many seconds (a educated guess that the call ended),the software will release the call and go to idle.
There are some external devices (channel banks) that will do that for you, but you will need a T1 or E1 interface into your router. That device will go in betwen your phone line and the router and provide that extra brains for you.
Good luck !!!!!!!!