Hi Jacky ,
As far as I understand your question, you are expecting a GUI from the
example, dial_tone.py. But gr.flow_graph is a used for connecting the
different signal processing blocks, and ensures a proper data flow from
the source to sink in the dial_tone.py case. Thus, the target of the
program is linking the signal source with the audio sink. So, since you
hear a sound ( a dial tone), the program executed successfully. If you
are looking for a GUI, you can install GNU radio companion(GRC). Then,
you can play with the different signal processing blocks. I think I have
answered your question.
Fasika