In 2.10 if I wanted Extension 6901 to show Extension 2000’s VM I would put this in the Mailbox field in the Extension settings of 2000:
2000,6901@default
In FreePBX12, how do you get one extension to show another extension’s VM?
Setup a BLF with *982000
The context for voice mail is different, and I’m not sure you can do two accounts at a time any more, but other than that, you should be able to manage this in the Extension settings.