Heres a simple circuit that takes a audio signal(from ham radios,computers or cd players) and converts them into a telephone signal. It powers the phone line with 22vdc(It doesn’t have to be 22vdc, different voltage means different resistor is needed). The phones can run as high as 48v. If you use cordless phones the converter will still need power. WARNING DO NOT CONNECT THIS TO A REGULAR TELEPHONE LINE OR LANDLINE THOSE LINES PROVIDE AC AND THE CAPACITOR WON’T BLOCK THE VOLTAGE. THIS CIRCUIT IS TO TAKE AUDIO FROM THE HEADPHONE JACK AND INJECT IT INTO CORDLESS PHONES WITHOUT A TELEPHONE COMPANY. I use mine to listen to IRC chat 1000ft away from my computer. This could also work with corded phones. I believe you connect them in series, where the lamp is. I don’t have any corded phones to experiment with, the resistor may need to be lowered for 2 corded phones. I tried connecting two cordless phone bases together but the handset said No Line and also you could hear them or hear the tone when you press number buttons. When I made this circuit the No Line message went away and you could hear the computer and tones for the number buttons could be heard.
The capacitor should be no more than 1uF of capacitance and non-polarized. And yes the cord is safe to touch since its not hooked up to a real phone line. The resistor should be 1/2W instead of 20W. I had extra of those so I used it.
In the circuit diagram the 44KHz is the Audio Signal and the lamp is the phone
The way I wired it. Part of the audio cord is behind the RJ11 and the other one by GND 
The schematic
I have two cordless phones but no splitter or second RJ11 connector. I will try talking over them when I get those stuff
