If your Samsung I537 comes from MetroPCS USA or T-Mobile USA then it's most likely your Cell Phone is locked with "Device Unlock App" and not asking for a unlock code (SIM Network Unlock PIN). In this situation it's very important to order the unlock of your device using this special service:
With all Samsung I537 Unlocking Code orders we usually deliver a complete set of unlocking codes. Most of the time you just need the Network Code (NCK) but when needed you'll also receive Unfreeze Code, Reset Key, or Service Provide Code. In the same email that includes your unlock code(s), you will also receive complete instructions on how to use it. Usually, you just insert the SIM card in the phone you want to use and the phone will prompt you for the unlock code. In case you need any assistance, our customer support is always there to answer any questions you might have. We will get back to you within less than 12 Hours from Monday to Friday.
1. Make sure that the phone PROMPTS for the code with a NON accepted sim card and that your IMEI is correct by dialing *#06#
a. Go to the Dial pad and dial *#197328640#
b. Please click on number  UMTS >  Debugging screen >  Phone Control >  NETWORK LOCK
c. You will see 3 options
d. Please click on number  Perso Sha256 INFO, the screen will change slightly
e. Please hit the MENU button then when the menu pops up please carefully hit BACK to return to the previous screen
f. Next Please click on number  Perso Sha256 ON, the screen will change again to a one line message
g. Restart the phone on the screen by holding the power button and click RESTART
h. When the phone prompts for the unlock code simply enter the unlock code provided to you
i. Your phone is now permanently unlocked!
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