Whenever I call someone or they call me my LG Vortex screen goes blank. I can still talk to the other person but when they hang up (there isn't a way that I can) the screen is black. I can't press any buttons that will make it go to the lock screen it just stays black. The first few times this happened i just took out the battery. Now even that doesn't always work. I had to hold down the lock and down volume button. Please help this is getting really annoying.