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.