iPhone voicemail help

I do not know what the password is when requested to hear voice mails. they keep stacking up and I can't delete them when i click on the phone icon, then see a red superscript number on the voicemail icon. when i click on it, the next screen is a keypad and it dials something that asked me for my password. There is the problem, I have no idea what the password is and cannot find out how to delete it or change it so I can get rid of those old voicemails.