Samsung Galaxy Grand Prime has many options to adjust your device volume
and improvements of audio quality of phone calls. 
You can adjust volume during a phone call by pressing the volume keys on the left side of Galaxy Grand Prime.
You can find more options by clicking on the 3 mini circles on your screen. If you want to further improve the quality of phone call, you can use the on / off function of noise reductions feature.
This is available in the More Options Icon (3 mini circles) then Settings and unmark the noise reduction check box.