Without know the signaling and such I can only offer generalities here. In general, it is not the CMS that initiates the disconnect. Typically it comes from the line side (E1/T1) through standard signaling like an ISDN disconnect. Unless you are using a channel bank with loop start. That may be a different story. Or the disconnect is coming from the IP side as an H323/SIP disconnect. In either cases, when the CMS receives these messages, it will pass them along to the other end to disconnect the line.
If this is easy to reproduce, you should probably run some standard event logs to see where the message is coming from (if at all) and what happens.