Phones configured as Ring Central extension should not need a pin code to access voice mail for that extension. (A pin code isn't necessary for an app on a mobile phone). The benefit is that voice mails can be quickly accessed without having to remember and input 6 digits.
You are correct that the applications do not need the passcode. This is based on the assumption that there is a passcode on your phone or computer and the apps do need authentication to log in as well. Physical phones not having these protections require the pin for the security of the of the messages.