Bad iPhone 5s quality of calling, what to do about it?

So I've brought a new iPhone yesterday, and it's perfectly fine, but when someone calls me or I call someone I can't hear the other person very well. I can hear it really silently. How to make it louder?

Added (1). Tried to increase the volume already!

I don't know how to fix it but you can just go to a Apple store and explain the problem and they might replace the phone or fix it for you in store

Look for the volume controll

Increae volume

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