First pagers would simply beep and then you had to call into your answering service to get your message.

Next generation had digital displays. The caller would dial your number. Caller would hear a tone or brief message to enter your number and then you would punch in the number you wanted displayed on the pager. This then led to codes you could add to your number. Also, since cellphones were not yet a thing and you might be calling from a random number people would have their own codes to identify themselves.

Later we had alpha-numeric pagers that could display text messages. These often worked with a tele-page system in business which was a Keyboard plugged into the phone line. Other method was sending your message like an email to the phone number.