It's pretty weird that this is only visible in the Obihai device's web UI?
Seems to be a fairly common cause of "stuck voicemail indicator"...and no idea why it seems to get checked/enabled on its own because my voicemail indicator worked fine until last week when this got mysteriously checked/enabled...
Per the Admin Guide:
MessageWaiting
This is a state rather than a configuration parameter, that indicates if there are any new messages for this subscriber on the service provider's voicemail system
I take that to mean it's read only, you get to see what it is set at, but you don't get to change it.