your issue is that you need to change your
Call processing T303 interval
Call processing T308 interval
Call processing T310 interval
Call processing T313 interval
timers to 60 seconds.
these are your ISDN timers that control how long they wait for proceeding and progress messages.
for some strange reason Clarent set them at 10 and 15 seconds on default. Simply change the setting and you will be good to go. Let us know if you need more help, glad to help.