I feel kinda silly but the answer MAY have been here all along. Somehow I missed this
http://forum.ppcgeeks.com/showthread.php?t=11482 . I have the same registry subkey and it's value is 100. I googled HKEY_LOCAL_MACHINE\SOFTWARE\OEM\Bluetooth\PhoneVol and got two DIFFERENT hits. Both suggest a value of 255.
It seems like a quantum leap from 100 to 255. Has anyone tried it ?