When you sign up with VOOV phone number, the default display name is your number until you update your profile.
Tap on your profile inside VOOV.
Select the edit icon (pen and note) in the top right, to edit your VOOV account details: Name, Profile Photo, Gender, About Me.
Logging in with Facebook, WeChat and G+ will use the display name you use on those social networks. You can also edit these.