Hello Burly:
In the voice mail programming when you are setting up your greetings and your custom service menus, you can assign a button as "voice mail service." For our business, I have the * key set as "voice mail service." Whenever a user calls in and hears the first greeting, he/she presses * followed by their mailbox number and password.
If you don't want to program a button, you can tell your users that when they hear the company greetings come on, they can enter #6* followed by their mailbox number to gain access as well.
Hope this is helpful.
-Michael.