Facetime Audio Calls Using iPhone sound issues

For anyone who has recently upgraded to El Capitan, and is using iOS 9.2.1 on an iPhone: You may find that when attempting to make or receive a phone call via FaceTime Audio (On your Mac), The caller, or recipient can hear you, but you can't hear them.

I had this problem both on my MacBook Air and iMac. Simple fix, reboot your iPhone. That solved it.