How to Disable needing to enter Pin to access my Voicemail every time?

I like to disable needing to enter my Pin or Password in order to listen to my mobile phone Voicemail every time? I try doing so in the adminstrative options from the voicemail options, but there does not seem to be such option. Every other mobile service I had in the past does.

How do I do this?

Thank you for your help.