asterisk users - Mar 2006 - Calls not tearing down properly

I have the following configuration:
 
Bell CO. <---ISDN---> Definity <---ZAP PRI--->  Asterisk  <--ZAP
PRI--->
Nortel BCM.
 
With SIP users on the Asterisk box.
 
I have found a problem with call tear-downs.  When a caller calls in from
Outside, or from a Definity station to a BCM station, _and_ the BCM hangs up
first, the Definity - Asterisk call does not tear down properly.  I get a
problem tone on the Definity station, or a "not in service" message
from
Bell for outside callers.
 
All other calls tear down properly, in either direction, no matter who calls
or who hangs up first.  
 
I have been having problems with my Definity Asterisk connection dropping
its D-channel (and all connections existing at the time of the D-channel
drop).  I believe that they may be linked.  In using gdb, I was seeing
problems with pri_disconnect_timeout.
 
Please help.
 
Regards,
 
Mark.

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