ISDN line is configured to drop when the user is not using the connection
ISDN lines are usually configured to drop after x minutes of inactivity to reduce the cost of the line.
When IP (ICA) traffic resumes, it is probable that the TCP session will time out before the reconnection is established and the Citrix server will be "unavailable."
If the user places the ICA session in the background (does not enter mouse or keyboard input) and works locally on the LAN, the ISDN line goes down and the company saves money. Pinging the server prior to going back to the ICA session should reestablish the ISDN connection. If the user then runs a batch file that pings the server x times before attempting to go back to his ICA session, the ISDN line will come back up, the TCP (ICA) packets will be able to travel across the line, and the ICA session will resume.