Nanatsu no Ko

Nanatsu no Ko (七つの子, lit. Seven children or Child of seven), is a popular Japanese children's song written by Ujō Noguchi (野口雨情 Noguchi Ujō). Renya Karasuma, the boss of the Black Organization, uses this song to encode his mail address.

Name
The name of the song can be validly read in two ways: seven children or child of seven (meaning seven year old child). Although not a literal translation, the song name is occasionally romanized as Seven Baby Crows or The Crow's Seven Chicks.

History
Ujō Noguchi wrote and published Nanatsu no Ko in Kin no Fune (The golden ship) magazine on July 1921. Nanatsu no Ko is used as the departure melody at Isohara Station in Kitaibaraki.

Plot
Conan hears Kir dial the boss's number.

Vermouth arc
Conan first heard the boss's mail address when Vermouth texted the boss after the showdown with her. He didn't recognize the tune at first although he thought it was familiar sounding and had a sad feeling to it.

Cellphone arc
Conan noticed that the first few notes sounded very close to the area code for Tottori prefecture. He guessed that it was a song starting with notes "B A G A". He discovered the song Nanatsu no Ko matched the sound of the key presses after asking Kyosuke Haga‎ what songs start with "H A G A" (B is substituted for H in German).

The phone number follows the tune of the first few notes "Mother crow, why do you cry?" of the song. The area code phone number for Kurayoshi and Yazu in Tottori Prefecture, 0858, do sound like the "Mother crow" part, but "Why do you cry" doesn't work. The notes on a cell phone are FA for 1, 2, 3; SOL for 4, 5, 6; LA for 7, 8, 9; and SI for *, 0, #. Each note gates slightly lower as you move to the left on the dial. So if we start with # and not 0, and don't care if the notes are slightly off, we get the email address #969#6261, which was purposefully made non-functional by Gosho Aoyama to prevent prank calls.

Kir arc
He heard Kir dial the boss's number and discovered she was a member of the Black Organization. Later Eisuke Hondou, who knew the number by the tune because his father used to text the boss, heard someone in the hospital where Kir was being held dial it. Ran relayed this information to Conan who learned there was an undercover Organization spy among the patients looking for Kir.