Low call volume on iPhone 8.

My iPhone 8 call volume is most low. I have to put the speaker on to listen clearly. I have one iPhone 6s as well which has very clear and loud speech .What could be the cause ? I have tried increasing volume with the volume rocker .Again tried making Noise cancellation on and off .Also tried disabling the "limit the loud volume with headset "setting. I have not tried resetting all settings .IMy phone is on iOS 14.8.