It’s a specific 56K protocol. There were a few different types of 56K modems and they did the last part of the handshake differently. One did the “boing boing” and another common one had more of an ascending tone at the end of the handshake.

You can hear the boing boing one at the end (1m54s) of that example www.youtube.com/watch?v=xalTFH5ht-k