why i keep hearing my own voice when someone call me sometimes?

[iMore Question]

when someone call me when i'm using apps like facebook or twitter.. i answered and heard my own voice.. but if i end the call and close all the app then she/he call me again, i can talk normally. i'm using iphone 6s.. is it normal or there's something wrong with my phone? if there's something wrong, how can i fix it?