i think it a problem with the sim card/OS.... when i used to use my Redmi 1s i had the same prb.... i used to call someone and i will hear nothing and call will end.... i had to reboot my phone each time this happened.... the same thing happened to one of my friend as well.... for me the prb got solved when i used a custom rom.... u can change the sim or reboot ur phone each time the prb occur if this solves the prb or install customs os like cyanogenmod or mokee rom....