If your ZTE V821 is not working correctly, you may need to master reset it and also clear the cache partition.
These two resets clear different parts of the ZTE V821 storage. Unlike a master reset, wiping the cache partition does not delete your personal data.
To wipe the cache partition, follow next steps:
Turn off the ZTE V821.
Press and hold both Volume Up and Volume Down keys at the same time.
Press the Power key only until the phone vibrates once, then release only the Power key.
Continue to hold the Volume up and Volume Down keys until the Android System Recovery screen appears.
Press the Volume Down key to highlight wipe cache partition.
Press the Home(GB only) or Power(ICS only) key to select.
Make sure Reboot System Now is highlighted and press the Home or Powerkey to reboot.
For more information: