A pager is a small, handheld communication device used to receive short messages or alerts (mostly the phone number and/or name of the person wishing to receive a call back from you).
Typically about the size of a matchbox, pagers notify users through beeps, vibrations, or text displays.
They were widely used before mobile phones, especially by doctors and professionals needing reliable, one-way communication.
The user would usually see the caller’s number and then use a conventional phone to call back the caller.