Most guides about writing numbers in words focus on money and dates. But there is a whole category of numbers most of us write every week: phone numbers, PINs, ZIP codes, and account numbers. These follow a completely different rule — and getting it wrong causes real confusion.

The rule: read them digit by digit.

Unlike amounts of money, where 1,250 becomes "one thousand two hundred fifty," a phone number like 555-1234 is never "five million" anything. It is "five-five-five, one-two-three-four." Each digit gets its own word, spoken in sequence. That is the whole rule, and it exists because nobody needs the mathematical value of a phone number — they need to copy the digits exactly.

The same applies to ZIP codes and PINs.…