When you restart your iPhone 4S, it will prompts you to unlock your SIM card. How to unlock this features can be found here.

To unlock the SIM card in iPhone 4S, you need the secret PIN and PUK numbers that you have been sent separately to the delivery of the SIM card.

Unlock iPhone 4S Sim Card – How it works
To start the unlock process, you need to switch to insert the card into the iPhone and the iPhone 4S. Then displays the message locked SIM card in the display.

“Now press the button” Unlock and enter your 4 digit PIN. Pay attention to the accuracy, namely you have only 3 attempts. Then you can only help the correct PUK.

“If you entered the correct PIN, you must still complete the transaction with” OK to confirm. Then, the SIM card is also unlocked immediately.

# Tip: You can disable the PIN entry when 4S also iPhone, so the next time you switch on any PIN. You should then set up a code for safety-lock, so that in case of loss no one can access your personal data.