Nortel Remote Office 9150 Installation And Administration Manual page 58

Remote Office 9150 description
Example 1
If the call lasts for 20 seconds and no other calls are made, the ISDN connection
drops when the minimum call duration timer reaches 59 seconds. The minimum
call duration timer expires before the idle timer.
Example 2
If the call lasts for 65 seconds and no other calls are made, the ISDN connection
drops after another 60 seconds has passed without activity. Since the ISDN call
exceeded 59 seconds, the minimum call duration timer no longer applies. The
idle timer is used, in this case, to prevent further ISDN charges.
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Standard 2.0
Remote Office 9150