If I understand correctly, the volume is low on phone calls. There are volume levels for the ringer which is separate from the volume of calls. It may just be the volume of a call that is low. I would recommend you read this article and use it to help troubleshoot the issue.
Turn up the volume
While you're on a call, adjust the volume with the volume buttons. Make sure that you can see the volume indicator on the screen when you press the volume buttons.
If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone
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