This is the method most users search for initially. These web-based tools claim to reverse-engineer the unlock code based on the IMEI.
When you purchase a ZTE modem or dongle from a network carrier (such as Vodafone, T-Mobile, AT&T, or Three), the device is often subsidized. To ensure they recoup their investment, the carrier locks the firmware of the device to their specific network. Zte Modem Dongle Unlock Code Calculator -16 Digit- Online