A standard dial tone consists of a 350 Hz and a 440 Hz signal. The generation of this combination of two tones provides a simple example of writing a Python script using the GNU radio software. The script is called dial_tone.py . Download the script and run. You should hear a dial tone. CTRL-C will stop the program execution. This script also captures the 350 Hz signal to a file. Instructions for plotting this data file in Octave are found here.