iphone 7 plus mic problem

other people can't clearly hear my voices during phone call...tested the guide posted by website also didn't help....only during phone call can't hear clearly...other than that is good....any way that i no need go to the apple center and solve the problem ? or just want to know how much will charge by apple center for repair the phone?someone tell me that is the mic ribbon problem....i not sure so whether is right or not? my phone is still warranty....thanks