2600 hertz

[1] Tone signaling carrier systems operated in the standard telephony voice frequency range (300Hz to 3500Hz).

The tones were typically transmitted in the same physical and logical channel, which characterizes these systems as in-band signaling methods, which do not require additional bandwidth for control of the network and benefit from a single amplification facility for speech and signaling.

[3] The in-band signaling method was vulnerable to talk-off conditions when the voice of a telephone user accidentally or intentionally generates the same tone or sufficiently strong spectral content at the frequency of the signaling system, a condition also known as falsing.

[5] The discovery of this phenomenon by technology-curious individuals in the 1960s, led to the abuse by phreaking, a subculture that exploited the technology to explore national and international telephone networks and place cost-free long-distance telephone calls.

The development of the T-carrier system in the 1960s helped obsoleting single-frequency (SF) signaling.