What they are doing is using software on the phones to encode/ decode by some protocal into what amounts to sound card input outputs. The audio then goes into the transmitting radio, and the other radio receives and converts it to audio which is then decoded. Radio Amateurs have been doing this type of thing for decades. A guy wonders how secure it is, when anyone can download the software. All you'd need if you are "spying" is the same software on a like Android device, and a radio capable of listening on that frequency. This has nothing to do with cellular as such.