Dial speed test
A dial speed test, also known as a dial pulse speed test, is a testing apparatus that measures the speed of a rotary telephone dial to indicate whether the dial is too slow, too fast, or the proper speed.
Dial speed tests were often used by repairmen to calibrate telephone dials to ensure that the dial was not too slow or too fast, as most electromechanical switching equipment had a limited tolerance for speed variances. A dial speed test typically provides a steady dial tone when accessed and is used simply by dialing "0".
Once the dial has returned to normal, the result of the test is indicated through a tone provided by the equipment. Typically, a busy tone indicated the dial was too slow, a fast busy (reorder) tone indicated the dial was too fast, and a steady audible ringback tone indicated the dial was within limits (typically 8 to 11 or 9.5 to 10.5 pulses per second[1]).
In some cases, the test results were conveyed by the number of tones, rather than the cadence. For example, dial speed tests used by Carolina Telephone used one beep for slow, two beeps for just right, and three beeps for too fast[2].