Right - wrong link...
1) Setup a DID (if you have one available) and route it to your VM extension (99,999,9999,99999)
2) If you use an automated attendant, have them dial the extension for VM when it answers
Then when it answers enter their extension (do not hit # here)
wait for it to ask for PIN, enter it and hit #
And so - use this:
http://www.3cx.com/blog/extension-user-manual/configure-your-voice-mail/
All this information is very easy to find on 3cx pages if you do a search.
Enjoy!