Optus is the second largest mobile device network in Australia. It offers a variety of phones and service plans to end users as well as acting as a wholesaler for service providers. Unfortunately, Optus locks the phones it provides so they can only be used on the Optus network. If your plan has expired and you've moved to another mobile carrier, you can still use the Optus phone by getting an unlock code and replacing the SIM card with your new network's SIM card. This is perfectly legal because you own the phone and you can unlock it if you wish.
Obtain the IMEI code of your Optus phone by pressing "#06#" on the phone. The IMEI code should be a 15-digit serial number. Write it down.
Navigate to a mobile phone unlock website. The links provided in the Resources section of this article provide a good starting point.
Enter the IMEI code you wrote down in Step 1 when prompted for this information at the mobile phone unlock code website.
Type in your country, phone network and your phone make and model when prompted. Provide your email address. The unlock code will cost anywhere from $12.99 to $24.99 as of February 2011. Click "Buy" after you've entered the information requested. Write down the unlock code.
Turn off the Optus phone and remove the SIM card from the back of the phone. It should be located next to or behind the phone battery.
Replace the SIM card that you just removed with the new SIM card from your new cell phone network.
Type the unlock code into the Optus phone and press "OK." The phone will reboot and display the new network at the top of the screen.