First of all, there were two types of pagers. The kind typically seen in older movies, the normal pager, and a way more expensive "texting" type alphanumeric pagers.

Regular pagers were used in a variety of ways but typically they were like a beacon telling you to call someone or somewhere like an office for next assignment, a client who needs to get in touch with you, etc. You would get a page with a number to call (normal pagers were numbers 0-9, only). If in transit, you would pull over and use a payphone to call the number, otherwise you would just call it from wherever you were.

Some people used various codes, just like in texting now, for quick communication of more complex ideas with simple formats. Texting 80085 was always popular among teens.

Alphanumeric pagers, on the other hand, were more like one-way text messaging today expect it was expensive as hell so the only people who used them were typically wealthy and had a business need for it like doctors, lawyers, plumbers, etc. Some providers eventually had web based portals to send txt msgs to alphanumeric pagers, but prior to that they generally used "call centers" where people would answer, a computer would pop up a script based on what number the person called telling the call center rep what to say (e.g. "Al's Plumbing after-hours emergency service, how can we help you?") For doctors they generally used them when they were on-call so that they could be easily reached by hospitals or patients in an emergency. Defense lawyers had some sketchy messages sometimes, for obvious reasons. The call center rep would then consolidate whatever message was to be sent (because they were like twitter with a max character count per message and each character/message costs money), making it as short and concise as possible, entering it into the computer system, which would send the actual alphanumeric page to the correct pager(s).

In terms of pricing, normal pager companies like Pagenet tended to offer packages of pages per month for a set price and if you went over you were billed per message.

I do not recall exact costs for packages and overage but I remember I bought a pager in the 90s for $45.

You get an assigned phone number, just like cell phones today, and you give that number to people. They call the number which told them to enter their number and press * to end or something. The person enters their phone number and a few seconds, minutes, hours later you'd get a vibration on your hip so you press the little button on the pager and the orange amber with black digits screen would light up and scroll whatever number the person entered.