This lock is a software restriction. When you insert a SIM card from a different carrier, the phone reads the unique identifier of the SIM and realizes it doesn't match the original carrier. It then prompts you for a 16-digit code.
If you don’t qualify for a free carrier unlock, reputable services like TheUnlockr, Cellunlocker, or DoctorSIM can provide a guaranteed 16-digit code for $5–$15. These are calculators in the sense that they query the manufacturer’s database, not guess the code. Zte Unlock Code Calculator 16 Digit Online Free
While many websites claim to offer a "16-digit ZTE unlock code calculator" This lock is a software restriction