Create all the extensions on the trixbox as your normally would and create mailboxes for each of them. It doesn’t care what you set for each mailbox as we will be pointing them all at the common box. For this example we will use mailboxes 300, 301, 302, and 303. Decide on what mailbox will be the common one. I am going to use mailbox 300 for the common one.
Now log into the trixbox and change to the /var/spool/asterisk/voicemail directory. You will see two subdirectories, default and device. Change to the default directory. Here you should see one directory for each mailbox that was created. The mailboxes do not get created until they get used. If you do not yet see one for 300 then this is because it has not been used yet. If it is not there, then just phone that extension and leave a voicemail message. If the other boxes 301, 302, and 303 are there, then you can remove them with the following commands:
rm -rf 301
rm -rf 302
rm -rf 303
The next thing to do is create a symlink so that the extensions 301,302, and 303 all point to the master mailbox of 300.
ln -s 300 301
ln -s 300 302
ln -s 300 303
Make sure the owners on the symlinked mailbox are the same as the master mailbox. They should be owned by asterisk.
chown asterisk:asterisk 301
chown asterisk:asterisk 302
chown asterisk:asterisk 303
next change to the /var/spool/asterisk/voicemail/device and make sure that the symlinks are created. If they are not, then manually create them as follows.
ln -s 300 301
ln -s 300 302
ln -s 300 303
You should now be able to have any of the extensions go into the common mailbox.