I often hear Japanese using a different method for saying a number like "248" as によんぱ — especially for highways and license plates.
While this one is easy to understand, there are others that I don't quite understand. Could someone provide a list of the different pronunciations of each number?
"2525" being ニコニコ is probably a good example of what I'm talking about.
1:い
2:に
3:
4:よん
5:こ
6:
7:
8:は、ぱ
9:
That's called [語呂合わせ]{ごろあわせ}; see the full article on Wikipedia.
Quoting from there:
1 : いち、い、ひとつ、ひと
2 : に、ふたつ、ふた、ふ、つ（英語から）、じ
3 : さん、さ、みっつ、みつ、み
4 : よん、よ、よっつ、し、ふぉ（英語から）、ほ
5 : ご、こ、い、いつつ、いつ
6 : ろく、ろ、むっつ、むつ、む
7 : しち、ななつ、なな、な
8 : はち、は、ぱあ、やっつ、やつ、や、やあ
9 : きゅう、きゅ、く、ここのつ、ここの、こ
0 : れい、れ、ぜろ、ない、わ（字形から）、まる（字形から）、おー（アルファベットのOから。）
There is no rule per se and an exhaustive list will need to be in a form of community wiki to be editable by everyone.
A mix of "on-" and "kun-" readings (without the last consonant) + some English pronunciation + some kana modification will work.
I'll list what I've heard with some example if possible: (I'm sure there are some commercial playing on these to advertise their phone numbers as well as a lot of other puns with numbers)
0: オ (0840 = おはよう), レイ, マル
1: ィ (0141 = おいしい)
2: ツ, 二
3: サン （～さん）, サ, ミ
4: シ, ヨ (4649 = よろしく)
5: ゴ, ィ
6: ロ (168 = いろは), ム (361 = さむい)
7: ナ (723 = なつみ)
8: パ (883 = パパさん), ハ
9: ク (931 = くさい)
10: ト, トウ, テン
As with @repecmps's answer, there's no real set rule, but often businesses will make up catchy words so that you can easily remember them. For example, the phone number 0840-0141 could be おはよう、おいしい to remind you of a breakfast diner. Also, people with the last name Saito (さいとう) often attach 3110 (3-さ, 1-い, 10-とう) to their email address, screen names, etc. You just have to be clever.
