Factory reset will do it - that wipes everything.
Or Settings > Apps > All, find "Contact Storage" and clear data. That will remove all contacts, but if they are saved as Google contacts and you have contact syncing enabled they will come back (doesn't happen with the factory reset because that removes your Google account from the phone as well).
Or open your contacts app and delete them from there one at a time (long press on the contact name and select delete). That will delete them on the phone and on your Google account. You could look in that app's settings to see whether there is a bulk delete option (I've never used a ZTE so don't know what exactly the options in its contacts app will be).