I had the same issue, one day my calls where just quieter. Tried updates, restart etc. But I found an effective fix. Once you are on a call, not on a headset, click the 3 elipses on the top right of the call controls and click "enable extra volume". Worked for me, call volume back to "normal". I guess I lent on the phone with my ear or something to disable to extra volume without knowing it.
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