This read-only value reports how many seconds the current ISDN call on this port has been up, giving a live call-duration timer for an active ISDN connection, relevant on ports used for dial-on-demand or backup ISDN connectivity. An admin uses this to see how long a currently active ISDN session has been running, useful for spotting a call that has stayed up unexpectedly long, potentially incurring unwanted usage charges, or confirming a backup ISDN call just recently connected. For example, if this backup trunk uses ISDN and lportISDNDuration shows several hours, the admin might investigate why the primary trunk has not recovered and terminated this presumably billed-by-the-minute backup connection.