2600
2600 is the frequency in hertz that American telephone companies used to play down the phone to when it was not being used. When the phone is being used there is other noise on the line, ring tone, numbers being dialed or talking. In the 1960's a free childs toy, a whistle, used to give a 2600 tone. When this was blown down the phone line when dialing, you could fool the phone into thinking that the phone was not being used and make free calls. The whistle was free in a a breakfast cerial "Captain Crunch", the name then became famous as a US phone phreaker took the name as his handle when he found out what the childs toy could be used for.
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