volume on iphone is low

Get more volume for what exactly. Volume controls on the iPhone, while it would seem is a single volume up and single volume down button perform differently depending on where you are on the phone and what you wish to do.

When on the lock screen and nothing else is open and you have Change with Buttons turned on in Settings>Sounds & Haptics, will raise or lower the ringer volume, which also can control notification volume and alarm volume. For volume increase/decrease in Music, you have to have that open and a song playing. For volume changes in videos, that video needs to be open and playing.

Now, given that information, what are you trying to increase?