Depends which 56K “standard” you are using. There were two before v90 standardised things, they had different handshake modifications for negotiating the faster link, and v90 was different again. Some ISPs supported only one of the earlier “standards” for some time after v90 (most v90 modems were firmware switchable between it and one of the older methods or were soft-modems where that switch was in the driver).
https://www.inetdaemon.com/tutorials/computers/hardware/mode...
My understanding is that it required digital equipment at the exchange end which would make it impossible to do with two standard modems wired back to back. Unless someone has some telco kit laying around, it might be a lost sound.
There are other methods but that's the one I would use. To see one of the other methods, try here: https://www.youtube.com/watch?v=yQ_hWS6ked8
V.90/92: https://goughlui.com/2016/05/03/project-the-definitive-colle...
X2: https://goughlui.com/2018/06/10/project-record-a-us-robotics...
K56Flex: https://goughlui.com/2018/06/10/project-record-a-k56flex-dia...
Turns out I've been trying to track down the USRobotics Sportster voice modem sound from your first link. Even better as I can't find my 56k USRobotics, only the 33.6k one, so probably wouldn't have ever found that sound with hardware either.
Thanks; that's been lodged in my head for a couple of decades.