How to block an iPhone 4 from calling certain prefixes? I have an iPhone 4 and I need to block certain prefixes between 1-5 digits.

For example, I don't want any outgoing calls to be made with numbers that start with "123", "456", etc, so I want to be able to block those prefixes so that any attempts to call a number that starts with those prefixes does not get placed.  Preferably, this list should be protected with a PIN so only I can alter it.

How can I do that?
There is no way to block outgoing calls on iOS itself, so your only choices would be to find a voip app on the app store that would allow that level of configuration or perhaps jailbreak the phone and see if that community has such a modification to the phone app.