[phire]

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.

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.