Solved! What to do when incoming voice we can't hear in phone call?

Do other sounds work? If you can hear other sounds, then there is something either going on with your volume (check the volume in call), your carrier/service provider is having issues on their network and you need to talk to them, or the person calling you has a problem with the microphone on their device.

However, if you can't hear even your phones sounds, ringing, etc., then you need to check the phone's sound levels. Should that not resolve it, then I would have the phone looked at by either the manufacturer, if it is under warranty, or a local tech.