Short answer - it depends. Depending on the server (each carrier has its own VM server), you might be able to do it with the VM number, a pause* or two, then the password (if it's numeric). Or use wait* and wait for the prompt to continue with the password.
*Dial the VM number, then hit the 3 dot menu and choose pause or wait, then the password. (You'll probably have to set VM up as a contact, then hit the star to put it into Favorites, or make a Direct Call widget to it.)