Dial-up internet connections may feel like a relic of the past, but modern telephony equipment makes it cheap & easy to set up your own dial-up ISP!
Forget about pricey interconnects with the phone company: with a few pieces of commodity hardware and an hour of setup, you can emulate a blazing fast 56 kbps network connection between two or more clients.
Despite variations in modem hardware, this guide should work with any dial-up modem that presents itself as a serial device to the operating system, whether it connects over USB, ISA, PCI, or RS-232.
Dial-up over VoIP can be unreliable so it is best to keep speeds low. If you don’t already have a modem I would recommend looking for a 14.4k modem for the sake of reliability.
The guide goes into detail on setup for Raspberry Pi OS.