It looks like others have requested something like this but they were asking to setup MWI for multiple voicemail boxes on a single phone but I believe this is more of a phone firmware feature. The solution below is a generic/universal solution that could work on any phone that has configurable BLF keys.
It would be nice if Thirdlane had an option to 'enable voicemail BLF' which would create a hint that mirrors a voicemail box status. FreePBX has something like this here:
For example x101 has their own voicemail box but is responsible for monitoring another staff member's voicemail while they are away (x102). By creating a hint (*98102) that 101 can subscribe to, x101 will be aware of when unchecked messages are waiting for x102.
Voicemail to Email is not a working solution
Subscribing to 101 only tells us when 101 is ringing but all we care about are the voicemail messages, so this does not work either.
If there are alternatives I am not thinking of please help me out.