My original N4 dropped, shattered and stopped working. So I bought another one on craigslist. It looks like its been rooted because its running cyanogen with a March build. I went to settings to do a factory reset but it remains cyanogen. My issue is everything works great except when it's making/receiving a call the screen goes black and i can't get it back until the call ends. I'm thinking about trying to get the phone back to factory settings to see if that will fix the issue but don't know how? Any suggestions?