I got AT&T Callvantage working now. Despite saying it is not able to registered my TA, apparently it works anyway (strange). I made some test call, including one to the AT&T service desk (a toll free number :-) and the sound quality is incredible clear! Woosh! I am impressed! :-)
Anyway, my US number is (650) 384-4515 so drop me a line if you like hear how it sound :-)