Switch ON your Nokia Lumia 635 phone with a non-accepted SIM card.
Nokia Lumia 635 will ask you to Enter Unlock PIN.
Input the 8-digit Unlock Code.
For Nokia Lumia 900 from AT&T USA, if code is NOT working:
The workaround is this: Enter the incorrect code until you run out of your ten (10) attempts. Then you'll get the *real* network unlock message, with 32 attempts. Enter the same 8-digit code you've been entering,
