We can make all extensions go to the same voicemail box by editing the no answer and busy settings though, that’s not going to give all extensions the message waiting indicator.
We just turn off voicemail for each extension and modify the follow me settings to send to extension X on busy and no answer. Still, we need a way to give each extension subscription to the message waiting indicator.
Instead of the MWI Button there is a solution with BLF: VM_200 (voicemail extension 200 as an example). This would make a BLF-Key red if there is a new message and you can listen to it.
That is the only alternative at this point.