One other thought just occurred to me--with voicemail tone set to ON...

Say you don't have a message, but MWI is on. If you connect to the VM system using the voicemail shortcut key on your phone and then hang up, might that trigger the phone to check for stutter dialtone? The point of this test is that you're essentially telling the phone the you're checking voicemail (by using the VM access hotkey), so it should 'know' to check for SDT after you hang up... I have no idea of Unidens are programmed to recognize that you've hit the VM hotkey, just a thought.