I like this explanatory graphic (after many many years of listening to the dialup sounds) - https://oona.windytan.com/posters/dialup-final.png

And this version combining the graphic and the sound used to make the graphic: https://www.youtube.com/watch?v=abapFJN6glo

And this alternative version (h/t @Kye): https://www.youtube.com/watch?v=jpMrTxMV6E4