You have the option during a live call to change how audio comes through your phone. The default options are Speaker and Phone (which is much quieter). If you have a Bluetooth hearing device connected to your phone, that will be an option as well.
To switch between audio output modes during a call
During a live call, tap the Audio icon on the bottom toolbar, to the left of the end call icon (see picture below).
Select your preferred sound output mode from the pop-up menu
Tap the caption screen and the selected sound output mode will be displayed
If you want your call audio on iPhone to always go over speaker, click this article to for set up instructions.