Hangup detection

Hello all, I am a new user just starting a new deployment. Things have gone well except for an issue with asterisk not recognizing hangups from our pbx (Toshiba DK280). What can we do as this is causing an excessive about of zero length messages, and clogging the v.m. lines, sometimes causing them to be put into an infinite loop.

Google the same subject, + asterisk if necessary. Also search for disconnect supervision. These are common problems and no easy answers.