HTB Crypto Challenge | Call

Hint: The flag format is: HTB{PASSWORD}

We're given an audio file called sound.mp3 that contains the very familiar sound of numbers being "dialled" on a phone aka DTFM (Dual-Tone Multi-Frequency Signaling).

I found an online tool and also a Github project that were able to accurately decode DTMF tones from an audio file, however, these tools only support .wav files so I started by converting the .mp3 into .wav with ffmpeg.

$ ffmpeg -i sound.mp3 sound.wav

So here's the output:

2331434783711923431767372331117714113

Then I distributed the numbers in the same cadence as the keys were pressed in the audio file:

23 31 43 47 83 71 19 23 43 17 67 37 23 31 11 7 71 41 13

At this point I was stuck.
I spent a lot of time trying to find a cipher that would fit this sequence, but finally it was a tip from the forum about looking for the "inherent mathematical properties" of these numbers that let me to realise that these are all prime numbers.

Finally, I used this Prime Numbers Cipher decoder to get the flag:

HTB{IKNOWTHINGSLIKEDTMF}