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I am very interested in doing a project to make this phone setup in this video. It essentially turns an old touch tone phone into a simple synth with an audio out and a power connecter. I would love to add it to my rig. I have a phone I bought off ebay and just need some advice on how it was made. Check out the video link below and tell me what I might do. I have been googling for days and this is all I could find on the subject. I even contacted the folks in the video. Not a peep. Any help is welcome. 
https://fb.watch/604fjfrXUv/ (https://fb.watch/604fjfrXUv/)