You need to find out what type of disconnect supervision your pbx supports. There are 3 main ones that come to my mind. Power denial (usually from telco), battery reversal (usually on pbx, but need fxo-m1), and supervisory disconnect tone (you have to put in the tone range the telco or pbx will play when disconnecting the call).
You can search for each of the on www.cisco.com.
Thank you,
-Mckee