They are using VoIP hardware, but it's not actually VoIP (which would be limited to 33kbps, they are connecting at 44kbps).
The 56k modem standard actually abuses the T1 digital voice signalling that telephone exchanges were using in the 90s/2000s. It only works when the ISP modems are connected via digital T1 lines, and the telephone connection remains digital right up until your local exchange before being converted to analog.
56k modems only work if one side of the link is digital, a pure analog connection, or any digital signalling that didn't match the exact standard telecommunication companies were using back then simply wouldn't work. So they have set up their VoIP hardware to convert directly from analog telephone lines to a bundled T1 PRI connection, which is fed into the ISP's modem bank.
It was a fun experiment done by a couple of people who acquired the equipment from a defunct ISP (and likely elsewhere) to see what could be done with period equipment. It's not a question of would you do it, but could you do it. Even after considering the cost of phone lines, the modems they used would have cost a few thousand and the multiple multi-port serial adapters weren't cheap either (they were planning to use two 8 port cards, which had conflicting drivers).
As for doing it over VoIP, the box they used appeared to be the sort of thing installed in offices to support multiple phone lines. I wouldn't be surprised if they could bypass the VoIP functionality altogether since they were making internal calls.
The key is to use a non-compressed codec such as G.711u/a-law, and disable echo and silence suppression. I can regularly get 28.8k-33.6k carrier speeds across the US when using an ITSP/VoIP provider and my modems connected to Grandstream/Linksys/Cisco analog telephone adapters.
There's a certain subculture who enjoys connecting machines to the modern internet which have absolutely no business being on the modern internet, and thus only have modems.
Dreamcast Quake 2 matches were one of the most popular uses last I checked
>"Business line" made the first one look like a loss leader.
That's exactly what it was.
According to monopoly regulation, the vast number of residential phone lines were intended to be subsidized by the lesser number of business lines from the beginning.
The 56k modem standard actually abuses the T1 digital voice signalling that telephone exchanges were using in the 90s/2000s. It only works when the ISP modems are connected via digital T1 lines, and the telephone connection remains digital right up until your local exchange before being converted to analog.
56k modems only work if one side of the link is digital, a pure analog connection, or any digital signalling that didn't match the exact standard telecommunication companies were using back then simply wouldn't work. So they have set up their VoIP hardware to convert directly from analog telephone lines to a bundled T1 PRI connection, which is fed into the ISP's modem bank.
They go into a lot of detail on this setup in "DIAL-UP goes DIGITAL: Setting up PRI for the TOTAL CONTROL - ISP Ep 7" -- https://www.youtube.com/watch?v=GQ0KTtMQ_8s
The earlier episodes are good too, but that one covers getting 56k modems working.