Before cell phones, which devices were used to send digital messages?

A pager (also known as a beeper or bleeper) is a wireless telecommunications device that receives and displays alphanumeric or voice messages. One-way pagers can only receive messages, while response pagers and two-way pagers can also acknowledge, reply to and originate messages using an internal transmitter. Pagers were developed in the 1950s and 1960s and became widely used in the 1980s. In the 21st century, the widespread availability of cell phones and smartphones has greatly diminished the pager industry.

Source: Wikipedia