I'm having very difficult time with keeping my Samsung Flight II unlocked! Every time I try to access the Voicemail, it locks on me & there would be no keypad to enter anything on....so all I can do is press & hold the unlock button on the side of the phone & then it still continues to lock. If it requires a unlock code, I didn't write mine down, so then, I'm "up a creek w/o a paddle"....I guess. I've checked all my phone settings, & everything is unlocked. What a mess!
I haven't used the samsung flight before, but I know with my phone there is a sensor near the top of the phone that triggers the screen to go dark when your making a call, so that you don't accidentally hit anything when you have it up to your ear. You may just need to clean that part of your phone so it's not getting a false signal.