low viber sound on receiving end

hi there, i,m experiencing peculiar type of problem in my redmi 3 pro. while making a viber call, the other end always complain about low sound, even they have full ear piece vplume on their's. But, its fine with other apps link messenger and imo. Its fine with regular gsm call as well. Is that due to low mic gain for internet calls? if it was the case, then messenger and imo should also have the same problem. i'm just confused.

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