Most of the time cell phones come with a lengthy contract and they are locked so that they only work on the one network. What happens if your contract ends and then you want to go to another provider? This is a good time to discover how to unlock an iPhone 3gs. This way you can take it with you and use it with any preferred cell phone service provider.
You should be able to call the network through which you first purchased the iPhone. Ask the company for the unlock code. They should be able to email the code to you along with the instructions on how to unlock it. When you get the instructions follow them carefully.
Now hold the power button down and slide to turn the phone off. Use a SIM removal tool to open the tray that holds the SIM card. When it opens take the SIM card out and then replace it with the SIM card provided by your cell phone network. Push the tray back in carefully and make sure that it is flush with the outside of the casing. Now simply turn the iPhone back on by pressing the power button. When it reboots it should be connected to the new network without a hitch.