As we know, the ADSL splitter box has 1 input and 2 outputs, one for 
phone signal line, and the other for internet signal line
I have executed many experiments with this device, and some give out 
interesting results. I list here 5 of them. Their schemas can be found 
in the attached screenshot
Experiment a)
Plug the telecommunication line into the input. Then in the output 
lines, I make the swapping. I plug the phone line to the internet 
output, and the internet line to the phone output
Then I dial the phone to call to a fax number
Surprisingly, I heard the busy tone, instead of fax tone.
Note: When I connect in normal way, plug the phone line to the phone 
output, and internet line to the internet output, this effect does not 
happen. That means I heard the fax tone as usual
Experiment b)
I make the looping back from the net output to the input. To do this, I 
cut into the cable and heal the wire inside, with the same color 
correspondingly (green to green, red to red)
Then, I connect phone output to a telephone machine.
Result: No tone heard when I picked up the phone machine

Experiment c)
I make the connection between phone output and net output, by cutting 
into the cable and healing the wire inside, with the same color 
correspondingly (green to green, red to red), similarity as I did in 
experiment b)
Then, I connect phone output to a telephone machine.
Result: No tone heard when I picked up the phone machine

Experiment d)
Wall line is plugged into the net output (Not the input). Then, I make 
the looping back from the phone output to the input. To do this, I cut 
into the cable and heal the wire inside, with the reverse color 
correspondingly (green to red, red to green)
Then, I connect input line to a telephone machine.
Result: No tone heard when I picked up the phone machine
Note: If I repeat the same experiment by healing the wire with the same 
color correspondingly (green to green, red to red), no issue happens. 
The tone can be heard from the phone machine
Experiment e)
Wall line is plugged into the phone output (Not the input). Then, I make 
the looping back from the net output to the input. To do this, I cut 
into the cable and heal the wire inside, with the same color 
correspondingly (green to green, red to red)
Then, I connect input line to a modem to emit Internet signal
Result: No internet signal is created