Not more SIP problems
I would...
1.) Start with a trace of the call probably monitor the port of the exchange server and see what is happening.
2.) The DND issues that I have seen and is very similar to your issue can be resolved by a couple means...
A.) Make sure DPNSS/QSIG Diversion Enabled is no in system options.
B.) Make sure public network to public network is enabled in the trunks COS.
C.) Most important of all set Suppress Simulated CCM after ISDN progress to no in the trunks COS.
This should fix it if not remember SIP = Stiill In Progress.