Lumia 1520 voice call problem

[Henky Foo]

Recently i bought a used lumia 1520, i am so happy bout this phone untill i found out a problem when making a voice call, the other side can't hear me when i am making a long word like hmmmmmm, ohhhhhhh, the sound will just go to very low level so they cant hear me.

First i thought that were mic problem, but after i tried using handsfree, the problem still exist.
Is it software? Or signal? Anyone facing this problem before

I am on windows 10 mobile build 586
Sorry for my bad english