WOOW!!! Thank you sooo much micland!! It works!!!
I tried a code similar to yours but some data were missing. Just needed to do a very tiny modification on your code:
or directlyqDebug() << value.variant().toString();
Except it, it is perfect! Thank you again.
I have no clue why the qdbusviewer of Qt 5.6 is unable to call the DBus method but the code snippet above is working fine.
I hope it is going to change as I intend to use it or the 5.7 version very soon.