It's been a while since I touched a 200 but if memory serves... If you have a console (SC1000), and you turn on COS option 321? (Transparent Multiconsole Operation), then you can view and delete message waiting from the console under the Att Function/ Stations Functions.
Also, if the set in question is a multibutton set 420, 4025, 5020, 5220 then the message can be read and/or deleted by pressing the message key while the handset is down.