For what reason does my iPhone 5 hang up after answering a call?

After receiving a call on the iPhone 5 I slide the slidebar all the way to the right to pick up the call.

After I slided I put my phone towards my ear but halfway there I can already hear the phones speaker making the sound of a call which is disconnected.

This happens frequently at any location which I call from. And I found out that my dad's iPhone 5 shows the same issues.

Is there any solution for this? Or is there something wrong with the phone?

My thoughts where that the sensior which checks either if your phone is in a closed case to shut down the screen is to sensitive and shuts down your screen the moment you bring your phone to your ear. Is this even possible?