Upon disconnecting, a modem does not drop carrier. Now we wait for the idle timeout, disconnect, and if the carrier is not dropped in 2 minutes time, again force disconnect and reset the port.

Fixed in RasExpress 5.61. Go to:
http://www.multitech.com/en_US/SUPPORT/Families/CommPlete4000/software and download 5.61 or newer.
Now we wait for the idle timeout, disconnect, and if the carrier is not dropped in 2 minutes time, again force disconnect and reset the port.