I just read through the manual for the a670 and have found the answer. What you need to do is go into your contact and edit the voice mail speed dial number. To do this find your voice mail in the contacts and press 'ok'. Next it will show you the code for your voice mail (*86). Scroll down and press the left soft button (right above the 'send' key.) Select the option edit or press 2 to get there. Now you are in edit mode. What you need to do at this point is make sure the edit cursor is at the end of the number. Then press the left soft button again which is indicating the function option on the
lcd display in the lower left corner. Then select the 'T pause'. This will insert a two second pause. I have 2 t pauses entered in my speed-dial. Next, you need to type in your 6 digit password or however many digits it is followed by the # sign. The end result will look like this:
*86TT123456#
So now you press 'ok' to save the number. Voila!

When you check your voice mail from the message menu (or by manually dialing from the contact list) you will hear the voice mail message and you will hear key tones as if you were entering the password yourself. No more need to enter your password to check your voice mail!!!
Hope this helps you out. If you still need help, see page 48 in your paper manual.