If phone is in a case, remove it and see if any better.
Also, make sure you aren't covering the mic in the way you are holding the phone.
It sounds like you might have an issue with the mic and something might be blocking it.
I'd suggest making an appointment with one of our gurus in store to have a look at it for you. You can book an appointment using this link https://guru.secure.force.com/O2DeskStoreLocator.
You can also book an appointment with Apple for them to have a look at it.
How old is the phone? If you're due an upgrade or close then it might be worth looking at changing it.
Have a great afternoon!
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