iPhone 5 drops calls occasionally around 10 minute mark? 3G issue?

I have noticed that when i recieve or make a call, sometimes the the other person on the other end can't hear me and will hang up, i look down at my phone and it has gone from Wi-fi to E (Edge - mobile network). I have looked in settings while on a call and noticed that the call will drop if you turn on/off 3g while on the phone.. Lots of other people have had an issue like this, and i'm just wondering why it's dropping calls and if its related to wi-fi changing to 3g or E at some point during the call?