You could really hear the noise with these however. And there's the fact that the speaker was directly touching the microphone. It's cool that this works basically inaudibly across a room.
You could hear them because they were constrained to the voiceband. Frequencies outside 300-3400Hz were deliberately filtered out by the telephone system to maximize the number of calls that could be carried on a limited amount of copper.
"Across the room" isn't really interesting, either. I've decoded PSK31 across a room. Higher, near-inaudible frequencies would probably make the decode even cleaner.
