Oh sure, it's for readability that you split it (or the equivilent when speaking) - but because the standard for splitting up a mobile number is the same as the

Oh sure, it's for readability that you split it (or the equivilent when speaking) - but because the standard for splitting up a mobile number is the same as the standard for splitting up a landline, it's pretty well known that, if a form (or whatever) were to want your number split in two, then you classify the first five digits of the mobile number as the "area code".