The first internet connections were dial-up
.Dial-up Internet access is a form of internet access that uses the facilities of the public switched network (PSTN) to establish a connection to an internet service provider (ISP) by dialing a telephone number on a conventional telephone line. The user’s computer or router uses an attached modem to encode and decode information into and from audio frequency signals, respectively.
Dial-up Internet has been around since the 1980s via public providers such as nsfnet-linked universities and was first offered commercially in July 1992 by sprint. Despite losing ground to broadband since the mid-2000s, dial-up may still be used where other forms are not available or the cost is too high, such as in some rural or remote areas.
You would have software on your PN (or on a mainframe) that would contain a telephone number. This software would contact your modem and dial the telephone number to connect with another computer. It is extremely slow. I can remember 300 baud rate at home and 9600 at work.