*exactly*a simple replacement code, but close. It's the number-part of this chart (the letter one's not so useful, since it just works out to be a simple replacement code and that's that. Might as well use Dancing Men or Pig Pen).

First, you do a little tic-tac-toe sort of hashmark/pound-sign affair and put the digits where they obviously should be. Chill on placing 'zero' there, since it's moot in this system.

Second, to determine what symbol goes with which digit, you find the digit in the hashmark and pick its section, just like Pig Pen code.

One tricky part kicks (as seen in Fig.3), where you add serifs (or lines, in the case of a 5 turning up) to the ends, with one set of serifs per multiple of ten. For 50, the line can go just about anywhere in there.

Obvs (in Fig.4), you just keep adding serifs/lines for each multiple of ten the root digit is being multiplied by.

[subhumans] Here's the fun bit! [/subhumans]

You take the symbols you've worked up, and then nest them. It doesn't really matter which ones are inside/outside which other ones, since you can tell the decimal place by the serifs/lines (Fig. 5a). In the case of a number with a zero in it, that's just skipped (Fig. 5b).

The problems are if you get more than four digits in a particular number, it gets really cluttery, you can't do anything that starts with a zero (so trying zip code 04401 is impossible), and things can get a little funky if you're dealing with 5059 or something else with multiple fives in there.

Frex, my Google Voice number (which has two fives in the final four digits):