With landlines, you know which part of your number is the area code, because it has significance. If you call people living near you, you don't dial that code. This behaviour doesn't translate to mobiles.
Even if you didn't conciously think this was the case, would you agree with me on which of the following looks the most natural?
07920 748555
0792 0748555
079 20748555
079207 48555If a website validates those two-box input fields, then surely it knows length of the prefix better than I do.
07920748555
When writing it you might choose to add spaces, but that'd be for readability, not because of some notion of area codes.
It's clear even from the name that area codes doesn't apply to _mobile_ phone numbers.
edit: and the grouping is as simple as you can do: digits are written in 5 groups of 2 digits.