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Well would you listen to those notes, that must be some long phone number or something!

Here we are given a ‘dialtone.wav’ file to download. This is an audio file, and listening to it sounds like someone is dialing a phone number.

We can use an online DTMF decoder this one worked nicely for us. The result is a long string of numbers.

13040004482820197714705083053746380382743933853520408575731743622366387462228661894777288573

Here’s some JS code that takes the decoded DTMF (bigint), converts it to hex, then decodes the hex to the flag:

var hex = 13040004482820197714705083053746380382743933853520408575731743622366387462228661894777288573n.toString(16);var str = '';for (var n = 0; n < hex.length; n += 2) {str += String.fromCharCode(parseInt(hex.substr(n, 2), 16))}console.log(str);

We were able to decode this into the flag using this site

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