Well from what I tried, I know of two options. Don't have the nav, but there probably isn't a difference with the Bluetooth modules.
1) Set up the voice mail number in the speed dial, call it, while you're on the call and it asks for a password, use the talk switch and say it, then it asks you if you want to send it, sent it with the off hook switch. Doing so grants entry into your voice mail.
2) Set up two speed dials, one containing your voice mail, one containing your password. While connected with your voice mail, speed dial the password number. works the same way.
3 (a guess?)) Set up the number in your phone so that it works with the pauses or waits and successfully grants access to your mailbox, and try sending that string to your nav unit.