In the last day, 3 friends with SPCS Sanyo 8200 and 8100 phones have
told me that their message waiting indicator has not gone away after
they had listened to all their voicemail messages and read any text or
Picture Mail messages.

I directed them to find the menu choice that clears the indicator.
On the 8100 it's: Menu | Voicemail | Clear Count.
On the 8200, I think it's Menu | Messaging | Voicemail | Clear Count