I have had a similar issue with my iphone 5. I had previously replaced my rear housing. I must have damaged the battery upon removal, it was very well stuck. The symptoms were randomly shutting down perhaps due to the battery playing up. It would read a certain percentage for half an hour and than drop by 25% and than die. I decided that the battery must have been damaged, I replaced it with a new one from ebay and everything worked well. I even had 30% battery at the end of the day. Goes to show the capacity drops a lot over a year. Here is where I get to a similar issue as is brought up in this post. I put my phone in my pocket facing down and when I pull it out.. It's dead. Turn it on and everything's fine. Hang it upside down its flat.. I shook my phone a little and noticed that something, most likely the battery was moving around in there! I would guess that in the down facing position it touches against something it shouldn't or tha perhaps the movement causes that battery to stop feeding power to...

I have had the problem for a good 6 months. Skype works, but not phone calls. I have replaced the earpeice, which interestingly fixed the issue for a few days and then crackling started to become an issue. So i had a look and decided to try replacing the antenna & ofcourse that didnt work. Also I replaced the bottom mic, I had no idea about the noise suppression mic at the top at the time. However I also have no idea what to do, you can replace the second mic, but its more money than I'm willing to spend for something that probably wont work. It could be some kind interfearence between connections, maybe, or the 2nd mic may not be working or at least its malfunctioning. Sorry I cant really help, I wish I could, I've spent a lot of time searching for a solution, but to no avail.