Since the PBX is initiating the calls, it is most likely a setting in the PBX. If the call flow were reversed, it would likely be a setting like 'isdn negotiate-bchan resend startup'.
In your Dchannel/Serial config, try changing
isdn network-failure-cause x
or
isdn disconnect-cause x
Where x is some trial and error to find a code the PBX will not want to hunt on. Good luck.
John Spade
webVoIP