Here is information on how to change where the audio from your iPhone is played:
Phone calls audio
In the call menu during a call, tap the top right icon, and select the audio output that you prefer. (see visual below or attachment):
You can choose "speaker" to route the calls to the phone speaker, iPhone if you want to hold the phone up to your ear, or your hearing aids if you want the sound to go to your hearing aids
Media audio
Scroll down from the top right corner of your iPhone to open control center, tap the little logo in the top right corner of the audio player, select where you want audio to go from the list (see visual below or attachment):
