You can “link” two mailboxes which reside on the same server by creating a symlink between them.https://www.voip-info.org/forum/threads/two-extensions-one-vm-box.3390/
For example, create two extensions each with mailboxes (extension 1000 and 2000). Dial *1000, wait for the voicemail greeting to start and then hangup. Do the same thing for extension 2000 (dial *2000). This will initialize the two mailboxes. Then from the shell, navigate to:
You will see a folder for named “1000” and one named “2000”. Delete one of them:
rm -rf 2000
Create a symlink from 2000 to 1000:
ln -s 1000 2000
That’s it! Any voicemail left in either mailbox will be available through either mailbox. MWI will also be reflected on both extensions.