These services have direct access to ZTE’s or SFR’s official database. They are not using a “calculator”; they query the real server.
If you are the original owner of the device and bought it from SFR, you may be eligible for a free unlock code directly from the carrier. zte sfr 101 unlock code calculator full
Hence, the need for an code.
Are you seeing a specific prompt on the screen? These services have direct access to ZTE’s or