Can you disconnect a call if it has been idle for so many minutes?

Yes, when the modem Usage is set to RAS or Modem Sharing. When the usage is RAS, the RADIUS attribute "Idle-Time" controls this function. This attribute must be defined in your RADIUS Server. When the usage is set to Modem Sharing, it is controlled by the "Idle Timer" option available on the Modem Usage page and is set on a per port basis.