How do you make the voice louder on an iPhone?

To increase the volume of the voice you're hearing on an iPhone, use the "volume up" button on the side of the device, which is marked with a plus sign.

The volume controls will increase or reduce sound of the operation the phone is performing at the given time, so to increase a speaker's voice during a call, the "volume up" button must be pressed when a call has commenced. Conversely, press the "volume down" button if the voice becomes too loud.

Make sure that a case or screen protector is not blocking the microphone or speaker grille, as this would negatively affect call quality.