I don't think that can be turned off. That's for security actually. If somebody else has your phone and you get a call, it goes back to the lock screen so that person cannot access your personal information
I don't understand your reply. I've had this phone for several months and it never did this before. I'm talking about how I initially unlock the phone and make a phone call. As soon as I hang up, it goes back to the lock screen. I could make calls back to back.. But now, if I need to make three calls, after each one, I have to unlock my phone. As I said, it never did this before.