Hello Gordo.
So, from the phone's dialer screen, hitting the menu then settings and unchecking "extra volume" didn't help?
Is the device in "driving mode" (basically louder as in speaker mode)?
One remedy reported is during a call first press volume UP a few times, then hold the volume down key.
I do see this issue reported around many forums. Trying different ROMs (rooted devices) gets mixed results.