Why can't I hear people speak on calls unless I use loudspeaker?

[AC Question]

If I try to make a phone call on my HTC One M8 I can't hear a dialing tone and when the person answers the call I can't hear them speak but they can hear me?

The strange thing is, I can hear them when turning on the loudspeaker?

I've looked for updates and done a full factory reset and neither have helped.

Any help would be greatly appreciated.