V8: Preferences - General Information(Section): MWI Notification
360/370: press "Settings" - Headset Device
Specify the type of MWI notification that will inform you when a new message arrives. A short beep <beep> is reminding you once on having mailbox messages waiting in which <reminder> is doing that repeatedly (3 beeps every 10 minutes). With <silent> this functionality can be switched off.