I wish to know whether it is possible to send data across PSTN network. While the call is in progress, I want to send some data through the telephone line (as DTMF tones can be sent). The data that I want to send is digital and is encrypted and modulated on an analog carrier (so that it can be send over an analog wire). I have also seen my broadband working in parallel when the call is going on.
I would guess PSTN work the same way throughout the world.