Apple and other highly branded companies are masters of spin. The glass isn't just half full, it's on the way to being filled. You can see that in action in the recently updated technical document titled: How to verify iPhone hardware is working correctly.
Now, if Apple were really honest about the content of this document, it would be called the iPhone Troubleshooting Guide. There really aren't so many "issues" or misunderstandings about a hardware product that's working correctly. This note is a list of ways your iPhone is working incorrectly.
Here are some samples of "correct" behavior:
Unable to make or receive calls. Battery does not charge from the iPhone charger. Can't hear through the receiver or through speaker, and can't be heard.
Actually, this technote is worth bookmarking. It's a very useful document with lots of step-by-step checks to diagnose hardware problems. For example, I like the procedure if callers complain about low microphone volume.
If you are using a third-party iPhone case, make sure it is not covering the microphone. Try making some calls without the case to see if a caller can now hear you more clearly.
If you have left the display's clear protective plastic film in which iPhone shipped, either ensure the microphone is not covered or remove it all together.
Don't laugh about the latter item. I've pulled off several from products, mostly phones, over the years.
People just forget about them or leave them on products. Or they don't have enough finger nail to get them off and then just give up trying. Then after a while when the soft plastic of the protector gets dirty or scratched they complain that they can't see the screen. Or in the case of the iPhone, they can't be heard for covering up the microphone.
These persons must have grown up in households where the plastic cover was left on the living room sofa to keep it "good" for when company comes. Perhaps manufacturers should make the plastic protectors less transparent and that might cause new customers to remove these covers.