I am able to find out where gives the problem. The problem is the difference of userid to excute "skype" and "Siptosis_linux" when I use VNC to do it.
After having changed to "root", they are able to function correctly as expected including echo test (both inbound and outbound voice packets) and skypename input bound and outbound calls.
It is because the default login for the VNC is not root. Now I have to switch the user to root first and followed by executing other commands. With this method, all the commands are executed under the same user (=root).