The dial tone on a telephone isn’t really a pure A, but a composite of two frequencies: 350 hertz, or cycles per second, and 440 hertz.
Most symphony orchestras tune to an A of about 440 hertz.
The tone was apparently not intentionally selected to make life simpler for musicians with no tuning fork, but rather evolved relatively early in the history of dial telephone equipment, when national standards were being set.
Various tones from about 160 to 480 pulses per second were needed for audible signals to subscribers and operators.
Some were interrupted to convey busy signals and other information.
When touch-tone service was introduced, the dial tone had to be altered slightly because it confused the touch-tone equipment, reducing the sensitivity of the receiver at the central office to the first tone dialed.
The new frequency is similar enough to the old tone to be accepted by customers as a dial tone.
