Thanks, TE. Makes sense. I did find the solution a while back, and I figure I'll post it here in case someone with a 3300 finds this by Google (as I did): By default, there is a setting on the Mitel 3300 under System Options called SUPERSET Callback Message Cancel Timer, which is set to 24 hours. This applies to all phones, and it cancels the MWI signal after 24 hours even if the messages haven’t been retrieved. You can set it from 1-255. If you set it to blank, the MWI will blink until messages are retrieved.