If your Samsung Galaxy S6 comes from 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 Galaxy S6 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.
EXTRA INSTRUCTIONS: If you get "Unsuccessful" again after type the NCK then:
7. Hit "DISMISS" button and compose *2767*3855# on phone keypad.
8. Wait for the phone to restart, this operation will erase all data from your Samsung Galaxy S6!
9. Retry the step 1 to 5 above !
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