Cannot hear the caller from iPhone 6S

Hi.

I have a new phone, the iPhone 6S.

I can't hear the speaker when I pick up the phone smoothly. Whit the headset is OK loud, but if I don't use the headset, I can't hear the speaker (very-very quietly).

Did you had a similar problem? What can be done for audibility? (menu item name?)

I have to sent back?

Thank for the help!

meliole