How to set retevis RT19 vox function?

Retevis rt19 small business radio, with CTCSS/DCS, VOX, monitor，Key lock，TOT functon, a few keys, very easy to operate, but how to open VOX Function?
VOX function frees your hand without pushing the PTT button for transmission. It uses voice to transmit, when the voice stops then transmtting stops automatically. The user can press and hold the side key "+” to tur on and enter the voice control mode, Then press the "+" and "-" keys to switch the level (off, level 1~9), press PTT to confirm and exit automatically.