IPhone 5 screen goes black after extended call?

My iPhone 5 screen goes black and locks after I finish an extended phone call, which is about 10-15 minutes, instead of launching back to recents. It doesn't do that with short phone calls. In general I just press the home button to turn it back on. It doesn't happen often, but has happened enough times to where its become an issue to me. I asked a an Apple rep at Best Buy and he informed me iPhones do this to conserve battery. But I find it unlikely, because my other iPhone didn't do that. Then I was told by someone else that if you're in a dark room, that throws the sensors off. I really don't know about that either. 2 out of the 4 times it happened I was in a dark room, what about the other 2?

Does anyone know anything about this particular subject?

How to delete songs on iPhone that have no information and don't play?

I recently purchased a few songs for a trip and duplicates showed up on my iPhone. However, these songs don't have any info such as album artwork, and they don't play at all. I want to remove them, but I have tried restarting my phone, removing all of my songs via iTunes and nothing is working. Help! My trip is in an hour!

Added (1). The duplicates on the phone don't even take up any space!

IPhone 5 data usage. How much data will I need for certain apps?

If I'm sending around 15-20 tweets a day, refreshing my twitter feed frequently about once every 10-15 mins & refreshing an Instagram feed around once an hour will 250MB's data be enough for a month?

I won't be posting any photo's on Instagram or Twitter when I'm away from Wi-Fi. I'll also be receiving about about 10 emails a day without attachments?

Facebook added contacts to my iPhone, how do I get rid of them?

I looked at my contacts on my iphone and I had like a million new contacts of my friends from facebook that I had never put on there. Facebook just randomly gave me a bunch of contacts that I don't even want. And I can't delete them the normal way. Why did this happen and is there a way to get my contacts back to normal?

How to use my iPhone abroad without being charged?

What I mean is how would I go about setting it so that I can use it as a camera and music player without any phone functions incurring charges - I need not to use data or to receive any messages or calls anyone send my way.

I'm locked out of my iphone

I've researched how to take the password off but I've read that I have to have access to my last computer I backed the phone up on and I don't use my itunes account anymore so I don't know my old password. Its truly a nightmare. Any ideas?

Iphone 4s battery goes fast but takes forever to charge?

So my Iphone4s battery drains extremely fast, but takes the longest time to charge. Maybe its because LITERALLY everything I do on my phone I listen to music. Even when I put it to charge I listen to music, so why is this?

IPhone vibrates twice when receiving a text?

Usually there's two quick vibrations that ring when you get a text. (Burr-Burr) And all of a sudden I get twice as many. It goes (Burr-Burr Burr Burr). How do I change it to once? It's annoying. It vibrates once when nothing's arrived, and then again when I actually get the text.