1

In many sources I have come across the explanation that 一 is pronounced yao1 instead of yi1 when used in things like telephone numbers. For example, my HSK textbooks use 一 (yao1), and I have seen it on ChinesePod. Here is an example this usage being described. I have seen and heard this consistently over the course of the last couple of years of learning Mandarin and assumed it to be correct.

I was quite surprised then when during the course of breaking down a character into its constituent components for learning purposes, I came across 幺 on HanziCraft. According to HanziCraft, this character is in fact the number 1 pronounced yao1 in telephone numbers. Further Googling revealed some other resources also using this character.

Which is the correct character to represent the number 1 pronounced yao1; 一 or 幺?