====== Whistling 2600 Hz ======

| **Field** | Phone Phreaking |
| **Went Obsolete** | Later in the 20th Century, possibly 1980s |
| **Made Obsolete By** | The introduction of Signaling System #7 |
| **Knowledge Assumed** | Absolute pitch or possession of a small toy whistle from Cap'n Crunch breakfast cereal |
| **When useful** | Illegally Dialing free long-distance phone calls (Also known as Phone Phreaking) |

In the early days, sending a 2600 Hz tone over the telephone is required to (illegally) make free long-distance phone calls (infamously known as Phone Phreaking).

The act of whistling 2600 Hz can be done by a person with absolute pitch (being able to accurately produce a musical note without the benefit of a known reference) or using the "Cap'n Crunch" toy whistle.

At one point in the 1960s, some of the Cap'n Crunch breakfast cereal included a small whistle that coincidently generated a 2600 Hz tone, which can be used for Phone Phreaking. Blowing the whistle with one of the whistle's two holes covered would produce a 2600 Hz tone.