770 should give you "Welcome to Merlin Messaging. Please enter extension number and #", NOT the automated attendant prompt, unless you've done some renumbering in the system, and the voicemail calling group is something different.

If you're calling from your own extension, you can press # instead of extension number and #, and if you set the minimum password length to 0, and don't set a password for the mailbox, another # will get you into the mailbox.

I would add that you should NEVER-EVER set the minimum password length to 0.