The price they charged to install a second one, "Business line" made the first one look like a loss leader.

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.

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.

They ran it themselves, you'd have to watch their other videos.