Out-of-band

In this additional spectrum, in-band signaling was sent down the wires outside the frequencies used for conversation. Actually, the signals were sent across the 3,500- and 3,700-Hz frequencies. Although these worked and were not in the talk path (out of the band) they were limited in the number of tones that could be sent.

One great way of solving the phishing problem is doing out-of-band authentication.

Defaulting to chaos still causes a lot of disruption, and would require out-of-band social coordination to resolve, but it places a much larger barrier in front of the attacker, and makes attackers much less confident that they will be able to get away with a clean victory, making them much less motivated to even try to start an attack.