Phone volume can barely hear when talking on phone

If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone - Apple Support

Is this a problem which has appeared on a previously working phone, or is it a new out of box phone with a problem?

Restart the phone to rule out software glitches.

Check the speaker is not visibly obstructed. Test as a speakerphone, and by playing a music clip. Ask Siri a question which gets a spoken answer. Compare the resulting volumes. That should confirm whether the problem is in the incoming signal or in the phone, then Contact Support if it seems the phone has a hardware problem.