asterisk users - Apr 2004 - Voicemail not hanging up?

For some reason after my users have checked their voicemail and hungup, Asterisk
keeps the channel open:

show channels
        Channel  (Context    Extension    Pri )   State Appl.         Data
      H323:1541  (incoming2  1            2   )      Up VoiceMail     u751
      H323:1505  (incoming2  8            2   )      Up VoiceMail     u758
      H323:1464  (incoming2  4            2   )      Up VoiceMail     u754
      H323:1425  (incoming2  2            2   )      Up VoiceMail     u752
4 active channel(s)

The users are connecting via H323.

Any ideas?

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