Why is the volume of caller low?

You can only increase the call volume during an active call, make sure it's set to the highest. Other than that speakers rarely stops working, Low sound is usually caused by dirt / dust trapped in the speaker grill at the top of the phone. You could get a cottonswab with some isopropyl alcohol and clean it. Best and proper way to do this would be to take the screen off and clean it with iso and add some dust blower to blow the clogged grill clean, you dont want to blow things into the phone because that could actually damage the speaker.