Websites like UnlockBase and Cellunlocker.net provide codes for a fee. These services typically require your phone's IMEI (find it by dialing *#06# ).

If you are the original owner and the phone was on T-Mobile , you can still request a 20-digit unlock code through their support channels.

Recent reports suggest some carriers like MetroPCS may no longer have the "software" to generate legacy keys even if the IMEI is eligible. Key Considerations & Risks How to unlock Nokia Lumia 521