asterisk users - May 2004 - Hangup problem during intergration with 3rd party pbx

I am trying to integrate asterisk with a 3rd party PBX for voicemail
(Mitel). For the most part, things are working well. We only have one
main issue left: hangup detection. The connection between the two is:

* w/T100P -> Zhone channel Bank FXO port -> Mitel ONS (station) port

(Yes, overkill, but we had these parts on hand so we didnt want to buy a
TDM FXO card)

The channels in * are signaled with fxs_ls signaling, as the PBX can
only hand out station lines with the hardware it has. So the problem is
when a call from the PBX into * (voicemail) is placed and the user hangs
up in the middle of the call, the channel stays in use until voicemail
times out. The docs from the PBX state that when a hangup happens, it
puts dialtone on the line, which is what is recorded in peoples
voicemail boxes when a call is hung up.

I have read all the mailing list posts about hangup detection with X100P
cards, even got out the volt meter and saw that the line does NOT drop
to 0 after a hangup. So the PBX is not acting like a true CO line, but
instead puts dialtone on the line. Is there a way for asterisk to detect
this dialtone during a call and hangup the channel? This seems to be the
only way we are going to solve this hangup problem.