2600 hertz (2600 Hz) is a frequency in hertz (cycles per second) that was used in telecommunication signaling in mid-20th century long-distance telephone networks using carrier systems. Tone signaling carrier systems operated in the standard telephony voice frequency range ( to ). They replaced direct current (D.C.) signaling on toll trunk lines because they could be used with any type of toll facility over any length of transmission line that was suitable for voice transmissions. This included transmission through line repeaters and other facilities that would distort, block, or otherwise prohibit D.C. loop-disconnect signaling, such as rotary dial pulses, and on-hook/off-hook signaling. Common frequencies for this purpose were , , , , and , the latter being just outside the voice range. These signaling systems were continuous tone methods, so that the idle condition of a trunk line could easily be detected by the presence of the appropriate signaling frequency, in contrast to conditions of a quiet line which could be due to pauses in speech, or arise from line faults. 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. The most common single-frequency signaling (SF) system in use in the United States from the 1950s to the 1970s used the frequency pairing of 2600 with 2400 hertz, while in other countries, some systems combined with a variety of lower frequencies in a compelled mode. Lower frequencies, such as , were used by the 1940s in composite signaling systems, however, the higher signal energy in speech at presented technical problems in protection against false operation. 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. In this instant, the call would be disconnected prematurely, and the trunk placed in idle condition. The Bell System in the United States used special signal-to-guard arrangements in the signaling receiver to detect this condition by comparison with the energy in the frequency spectrum outside the signaling tone. The condition was also often mitigated by narrow-band notch filters during the seized line state. The discovery of this phenomenon by technology-curious individuals