There are/were several types of pager codes in the 1990's before cellphones become common with inexpensive data/text plans.
The simplest (and prolly earliest) pager code was the Calculator Method. This method replaced letters with numbers that resemble those letters when held upside down (similar to how you'd create words using upside down numbers on a calculator). These were typed backwards so that the effect was easy to read. Examples:
- HELLO would read 07734. 4 kinda looks like an H, 3 is clearly an upside down E, 7 is clearly an upside down L and 0 is O in either direction.
- GUESS WHO would read 04177*553176. 6 kinda looks like a G, 17 looks like an upside down U, 5 looks like an S, * is a space, 177 looks like an upside down W.
Another common pager code system consisted of a string of numbers that were typically 3 letters long used to represent specific words and phrases. Though I doubt there is consensus regarding a name for the collection of these pager codes, these seem most closely associated with the term "Beeper Codes". Beeper codes were really a collection of many individual codes that were derived from several cipher methods. Many codes used letter count, but other methods were also employed.
- I LOVE YOU was commonly typed as 143. This required foreknowledge of the code, as any combination of words can share common letter counts. 143 could've easily meant I CAN'T FLY if you didn't know the established meaning.
- I LOVE YOU was also 831 from the phrase, "8 letters, 3 words, 1 meaning." Again, foreknowledge is necessary to decipher the code.
Number look alikes
The most versatile pager code was a slightly more formal substitution method. Letters were represented by a look alike number or string of numbers. This is similar to the Calculator Method, but can be read rightside up and used to spell any word without a lot of forethought (or turning your phone upside). Different varieties existed, but the most common was this:
|Letter||Number Look Alike|
|Spacebar||- or *|
With this method, complex messages could be sent without a lot of effort. Even after texting and cellphones become more common, this was still a good system to encipher messages from casual interloping.
- HELLO becomes 43770
- WISH YOU WERE HERE becomes 111154 4011 1113123 43123 (a special symbol for spacebar wasn't necessary once texting was available)
- FOUR SCORE AND SEVEN YEARS AGO becomes 401112 560123 8170 5311317 438125 860
Pager codes were still used by some people even after texting become available because most early texting-capable cellphones didn't have an alphabet keyboard. Typing words was still tedious using the texting system. It was prolly around the time when type-ahead appeared that use of pager codes finally become uncommon. Once smartphones become common, there was really no need for pager codes anymore at all, except for fun. I still find myself using some of the old 3 digit beeper codes if I don't feel like typing out a common phrase.