The Samsung SGH-X640 functions on a GSM network. This means your phone has a SIM card, which allows you to switch phones. If your phone is unlocked, you can change networks by changing your phone SIM card. Chances are your Samsung SGH-X640 is locked to your carrier if you purchased the phone under a contract. You must unlock your phone if you want to switch networks and keep your phone. You will need the phone's IMEI number and an unlock code to begin the unlock process.
Turn off the phone and remove the battery cover from the back of the phone. Take out the battery. Record the IMEI number located on a white label on the bottom of the battery carriage underneath the battery. Input *#06# to obtain the Samsung SGH-X640 IMEI number if there is no white label on the battery carriage. You will need the IMEI number to obtain an unlock code.
Contact your carrier to obtain an unlock code. T-Mobile will provide a free SIM unlock code if you have a past or current account in good standing for at least 40 days. Alternatively, you can purchase a code online from Super Unlock Codes or Mobile Unlocker or other sources. (See Resources.)
Remove your current SIM card and insert a SIM card from another network.
Power on your Samsung and enter the unlock code when "Password" appears on the display. If "Insert Correct SIM Card" appears on your display, enter #0111_unlock code#. Remove the SIM card and re-enter #0111_unlock code# if the unlock procedure is unsuccessful. Replace "unlock code" with the code you purchased or received from your carrier.
Check for a money-back guarantee before purchasing a code online.