2600 Hz is a frequency in hertz (cycles per second) that was used by AT&T as a steady signal to mark currently unused long-distance telephone lines. During the 1960s, in-band signaling was used, so the same line for both voice conversations and telephone connection management signals. Since a pause in a voice conversation would produce silence, another method was required for switches to determine available circuits. The solution AT&T created was to produce a 2600 Hz tone on idling trunks.