Please check with bubblestar who encountered the same problem. For me, I do not and I never play the skype at the server machine to test. I use asterisk softphone to test. No problem encounter but takes care on the ip and ports for connection.
Skype still has problem when you use on its own at the server machine. Only speakers work but not the microphone. It is reported that audio problem is very common in Skype for linux. Some may use modprobe snd-dummy to resolve it but I don't want to make things too complicated, so I keep it intact.
On the contrary, audio problem disappears when I use SipToSis. Echo123 is fine and dial out sound and voice are normal & clear.
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).