That is the enhanced audio feature. Been covered MANY times before. Depending on whether you have treble boost of bass boost, it will climb or decrease accordingly.
A new icon has started appearing when I'm on calls, just below the volume indicator (picture attached).
Any ideas as to where it came from?
Open the phone app (green button). Hit BB button>options>general options. Probably "Enhance Handset Call Audio" and/or "Enhance Headset Call Audio" has been changed to something other than normal.