[susam]

The phoneme-number assignment is something I learnt from a book. That system was developed by several people over several hundred years. See https://en.m.wikipedia.org/wiki/Mnemonic_major_system for more details about it.

Indeed /w/, /j/, and /h/ are not assigned any numbers in this system. I believe the intention was to omit sounds that are phonetically too similar to vowel sounds.

A word composed entirely of vowel sounds are not used in this system.

[schlauerfox]

Yeah, I leaned from an old "Bruno Furst" memory course, but it's fairly widespread to use the same system, because you can create visual mental pictures from combinations of the sounds into a word dictionary that links to numbers. The phonetics are fairly loose and designed to be adapted as you need it. You just insert vowels as you need to make words mental pictures vivid. Once you have a 'library' it becomes a hook system.