BTW: this may only be a problem in the event a call is transferred from the Mitel side tying up two trunks.
If this is the case there may only be a couple of solutions:
1) put a device in series with the trunk that listens for dialtone and disconnects the line after a few seconds.
2) disallow trunk to trunk transfers. Add a CO flash button the phone and transfer that way.
Ralph
(I've never used a CO Flash button so I'm not sure how well that will work.)