Was talking to my friend (native Chinese) the other day and when she read out a phone number she said "yao" for all the "ones" in the phone number.
However, as far as I remember, for all other cases she says it normally - eg. "yi ge" when counting etc.
Also, as far as I know, people from Taiwan don't use "yao" but stay with "yi" for all cases of uses of the number "one".
Does anyone know why "yao" is used instead of "yi"?