A chapter on memorising numbers from he book The Memory Book: The Classic Guide to Improving Your Memory by Harry Lorayne and Jerry Lucas uses a similar technique. They recommend memorising this mapping:

0: 's'

1: 't'

2: 'n'

3: 'm'

4: 'r'

5: 'l'

6: 'sh'

7: 'k'

8: 'v'

9: 'p/b'

To use it, just make images/stories that correspond to the digits you need to remember.

The more absurd the story, the less likely you'll forget.

E.g. pi (3.14159265359 ) could be the images of these objects: mit, rat, lab, Noah, shell, mail, bee. Then just sew them together in a story e.g. someone drops a mit, it lands on a rat, the rat runs up to someone in a lab, it's Noah, he throws a shell, it knocks over some mail which lands on a bee.

I don't use it often, but I like to memorise any 8+ digit door code when checking into a serviced apartment or airbnb, in case I forget my phone and would otherwise be without the code.

Funny, I just finished that book.

They were using a few more mappings per number for instance:

0: 's', 'z', soft 'c'

1: 't', 'd', 'th'

6: 'j', 'sh', 'ch', soft 'g'

etc

The problem I have, is that they present them from 1 to 0, and the first 4 have some mnemonic logic added to them: a bar in 't' for 1, two bars down in the 'n' for 2, three in 'm' for 3, and the 'r' from fouR in 4.

Great. And then mnemonic logic goes out the door. 5: 'l'. Ah. A single bar. Err.. is it 1 or 5..? 7: 'k'. Imagine the K looks like a 7. Ok, maybe.

So the mapping itself starts getting confusing. Unless you do what they claimed all along the book you shouldn't do: rote memorization.. Which was grinding my gears a bit..