Hearing Problem

Hello,

my friend bought an iPhone 6S+ for me around a month ago from Dubai.

I started using it in my country.

From the very beginning it has issues with hearing from the other side.

Whomever I call (different persons from different locations ) have hearing problem. Callers(my) voice fades out at times and then again it becomes normal. This has become a regular phenomenaon.

And also in addition to that surprisingly when I talk at times it gives a Hallow Feel into my ears ! (I m perfectly fine with no hearing problem).

My phone is still under warranty but I have no authorized store in my country . I will be going to Switzerland for only 3 days after a week.

suggestions (including Apple) for how can get rid of this problem/phone at the earliest would really be appreciated.

thanks in advance.

Regards.

A Devoted Customer