When on a call and it is connected, if there is a volume button on the side of the phone try turning that up, or moving the directional keypad (the up down left and right button) un down left or right you should see a screen come up when you do this the shows the volume going up and or down, if non of these work the best thing to do is go into a store with the phone, depending on how the phone was bought your brother might be useful when you go in along with your receipt.
Whether this can be fixed or not depends on how old the phone is and what the warranty is more warranties now are 12 to 24 months, i don't know what vodafone's are though
