This is the hardest part. If you don't remember which carrier originally locked the phone, look for a logo on the phone's casing or boot screen. Common codes:
Here are practical ways to emulate the look and function of the DCT4 calculator in a browser or as a tiny app: nokia dct4 calculator online
The phone should now display "Phone Restriction Code Accepted" or "Code Accepted." This is the hardest part
: Advanced users sometimes select the specific chip architecture (usually ASIC 2 for standard DCT4). Step-by-Step Unlocking Process nokia dct4 calculator online