Hard Reset ZTE Sonata 4G

First method:

1. Make sure, that your phone is off.
   
2. Now press and hold this buttons: Volume Up + Power key.
   
3. Next from the menu choose "Wipe Data/Factory Reset", using Volume buttons to scroll to option, and Power button, to confirm operation.
4. Then select "yes", and confirm with Power button.
5. After that press Power button to enter option "reboot system now".
   

Second method:

1. Power on the device.
   
2. Then press Menu key, and goto System settings -> Backup & reset -> Factory data reset ->Reset phone -> Erase everything.
3. Now your phone will reset and restarts.

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.