By default, with Siri on iPhone 4S you can compose and send an email, text and many other things all without entering your passcode credentials. I prefer it this way, but others may want some extra security.
Just follow the simple steps below to lock Siri with your passcode:
- Open your Settings App,
- Go to General
- Select the Passcode Lock
- Make sure the Siri switch is set to OFF.
This will force you to unlock your phone before using Siri.
Source: Macgasm
