The most common "story" behind your call barring password is that it’s a default safety code set by your network provider or phone manufacturer to prevent accidental or unauthorized changes to your calling restrictions.
Here is the information regarding that password: password for call barring
A call barring password (or PIN) is a 4-digit code provided by your mobile carrier to prevent unauthorized changes to your call restrictions. It allows you to block specific types of traffic, such as international calls or incoming calls while roaming, directly at the network level. Common Default Passwords The most common "story" behind your call barring