Sm-g3608

My previous phone was a blackbery and I used the phone for 3 years without it giving the problem that im experiencing with this phone.when im on a call the people im talking to cannot hear me and I cannot hear them most of the time I hear a sound like a scrachy cd.i bought this phone 3 months go and 3 days into using the phone I started experiecing problem I contacted mtn they confirmed that the problem its not with the network.i need you guys to please fix this and the reason im only complaining about this now its because my colleague is using the same phone and she confirmed that she is experiencing problem as well.