Locking down all apps except the phone/call app on the iPhone 4 running iOS6?
You can turn on Restrictions in iOS Settings (in the General section), which allows you to disable all non-standard apps, as well as some of the built-in ones (such as Safari, Camera, FaceTime, iTunes and Siri). Unfortunately, I believe a few built-in apps remain, such as Messages, Calendar and Maps, I'm not aware of a way to disable them completely.

A word of caution if you're testing this out—disabling apps, then re-enabling them will destroy your home screen organization.