I browsed internet after I posted the question. I came across a way to get around. after the usual *86 (to access voicemail) add , (means pause for 2 seconds) then key in password followed by # In that way, the password will be automatically keyed in each time you access voicemail. I tried and it really works.
However, it won't let me replace the speed dial 1 with this new number and I have to assign another speed dial number to it (that means the default speed dial 1 is useless to me now). Having said that, I am still very happy that I no longer need to key in password for voicemail anymore. Thanks a bunch.