There are two types of telephone volume on the iPhone.  

There is ringer volume and volume during a conversation.  

They are adjusted differently.  

You can adjust the ringer volume by pressing the “Up” button when you are not in a call.  

You will see the “Speaker” symbol.  

As you press the “Up” button the ringer volume increases.  

This action does not increase the volume during a telephone conversation.  

To increase the “conversation” volume, you must be “in a call.”  

During a phone call, use the same “Up” button.  

You will see the same “Speaker” symbol but this time the “Up” button turns up the volume during the conversation.

               The symbol in the middle of my “Home Screen” shows the highest volume available.

One other note!   If the volume on music, video, audiobook or YouTube is not loud enough, use the “Up” volume button to make it louder.