Some historically referenced (but now mostly defunct or unsafe) names include:

Stealing your phone's IMEI number for fraudulent activities. Safe and Free Ways to Get Your 16-Digit ZTE Unlock Code

ZTE has evolved its security over the years. Older models (like the ZTE Blade, ZTE Grand, or some ZTE Mifi units from 2010–2014) used a simpler, often broken algorithm that enthusiasts could reverse-engineer. For those rare legacy devices, free calculators did exist.