The phone's native "Settings > Phone > Call Barring" menu is slowly becoming a legacy interface. In fact, on many carrier profiles, this menu doesn't even appear on the iPhone anymore because the carrier bundle disables it in favor of their app.
The answer lies in the architecture of the GSM (Global System for Mobile Communications) standard. When SIM cards were first introduced in the early 90s, carriers needed a way to allow subscribers to control network services remotely. call barring password