Adjust Volume

• Adjust the call volume by placing or receiving a call and, while the call is in progress, press the "Volume Up" button on the left side of the iPhone. Alternatively, press the "+" button on the headphone controls on the headphones that came with the iPhone. Each time you press one of the buttons to turn the volume up, the call volume increases incrementally until reaching the maximum level.
  
  

Case Obstruction

• If the maximum call volume level still does not produce satisfactory results, it may be that your case is obstructing the iPhone's speaker. Check to see if your case covers the iPhone's speaker in any way. To test if the case is the issue, place a call with the case in place and take note of the volume level. Then, place a call with the case removed and take note of the volume level. If there is a noticeable difference, the case is muffling the sound or limiting the volume in some way.
  
  

Display Film

• Plastic display film may be obstructing the speaker. Remove any case on your device and examine the edges around the iPhone's screen. If there appears to be a plastic film over the screen, peel off and remove the film. Display film left on the iPhone can slide around and cover the ear speaker. With the film removed, replace your phone case if there was one and then check the call volume.
  
  

Lint

• If you are still experiencing problems, there may be lint blocking the speaker. Examine the call speaker located to the right of the front-facing camera. If there appears to be lint inside, use a soft-bristled brush (such as an unused toothbrush) to brush the lint out of the speaker. With the lint removed, place a call to check the volume.