How to force a factory reset or hard reset on ZTE devices

ZTE Groove X501 ZTE Groove X501
ZTE Kis III V790 ZTE Kis III V790
ZTE V887 ZTE V887
ZTE V889M ZTE V889M
ZTE Avid 4G ZTE Avid 4G
ZTE Grand S ZTE Grand S
ZTE V81 ZTE V81
ZTE Director ZTE Director
ZTE Blade C V807 ZTE Blade C V807
ZTE Grand Memo V9815 ZTE Grand Memo V9815
ZTE Geek V975 ZTE Geek V975
ZTE Blade III Pro ZTE Blade III Pro
ZTE Grand X Quad V987 ZTE Grand X Quad V987
ZTE Grand X2 In ZTE Grand X2 In
ZTE Grand X Pro ZTE Grand X Pro
ZTE Vital N9810 ZTE Vital N9810
ZTE Imperial ZTE Imperial
ZTE Reef ZTE Reef
ZTE Blade G V880G ZTE Blade G V880G
ZTE Blade V ZTE Blade V