The term dial-up originated with individuals and organizations that utilize the telephone voice network for data transmission. This connection method has been used for over 30 years. In the early growth of the Internet, the only common access method for the public was through an Internet service provider (ISP). Using a modem connected to any typical analog phone line, users "dialed up" the ISP via the telephone network and were connected through the ISP to the Internet.
In the really early years, the access rate provided by dial-up was limited to .1 to .3 kbps (300 baud), but it quickly evolved to reach speeds of 9,600–14,400 bps by the time Internet access started being available to the public (early 1990s). Modern modems support line rates up to approximately 56 kbps, with additional speeds available using different forms of compression.
Dial-up isn't just for Internet access; many organizations still use dial-up as a form of backup connection into remote sites. If a primary connection to a site fails, the dial-up line can be used to connect to the remote equipment for access and troubleshooting.
Source:
https://www.informit.com/articles/article.aspx?p=2459346
