Many carriers all over the world will network lock iPhones.
Step 1: You will need to contact your carrier to unlock your device. You can usually call them or request your device unlock online.
You will need an identification number for your phone, called the IMEI, which can be obtained by dialling *#06#
Step 2: Backup
and restore on iTunes.
After unlocking it with your carrier, please perform a Backup and Restore for your phone on iTunes. For the unlocking process to be complete, this step must be done. Please ensure that you have a SIM that is different from your original carrier inserted, connect the iPhone to iTunes, click on “Backup this iPhone”, and then “Restore iPhone”.
Once both steps have been done, you will receive a notification from iTunes "Congratulations, your phone has been unlocked"
Step 3: Restart your device by holding the home button and top lock button at the same time until the Apple logo appears. This will restart your device and when it is switched back on your iPhone should now be unlocked.