Your best, safe, and most economical path is:

on Google Play offer free calculation for specific carriers like AT&T (if out of contract) but may charge for "premium" statuses. Third-Party IMEI Services : Sites like Mobile Unlocked

Warning: Do not use a "calculator" to guess codes. If you enter a wrong code 3 times, many ZTE models will show "Code Error. Perm Failure" and lock you out permanently.

Finding a reliable 16-digit ZTE unlock code calculator for free online is difficult, as most free tools only generate 8-digit codes or support older models. Many modern ZTE devices, such as the Telstra T100, specifically require a 16-digit code that standard generators often fail to provide. JustAnswer Reliable Unlocking Options