Your built-in network channels support Gsm 900/1800, you need not to change it. If your handset is locked to a network, you can unlock it using different techniques (means you can use only one sim card); First of all type this *#2640# on the main menu, you'll see the lock code which is in use, but sometimes this doesn't work. After you get the number, note it down and insert a gsm sim card and when you are asked to enter the lock code, enter the code which you just noted. Now your phone is unlocked to any network, but there's one more thing, this will be required to type in every time you change sim cards. So you'll also need to completely change the number. To do this, go to Settings - Security - (Phone Security) - Change access code and change it to whatever you want. That'll be your code until you change it. If in case, the above code doesn't work for you, type *#7780# on the main menu. (note, this will delete all the contents of your phone memory, memory card and sim memory, including messages, contacts, photos, etc.. so please remove them before you proceed). This will unlock and your phone will become as it was manufactured. Your code will also be changed to 12345 (default code). If again, this doesn't work, please visit a mobile repairing shop and ask them to unlock your mobile. They'll use a small box (Ufs box or Jaf box) to unlock your phone using a data cable (not usb cable) through the computer with an unlock software or bbf-5 program. You'll loose nothing, but the only thing is, they'll charge for the service. Your phone will reset to the original settings with the default code 12345. Good luck. Thanks for using FixYa.