What country are you from? In the UK and US area code is pretty obvious to everyone (even in the UK where the "area code" of a mobile is basically just how we split the number based on landline numbers, despite mobile numbers not being geographically based).
I'm in the UK, and I don't think it's obvious what part of a mobile number is the "area code". With a phone number of 11 digits, how many digits form the area code? In a world where numbers aren't even connected to specific provider, the concept of area codes for mobile phones is broken. (Technically, my area code should start with +44 as well, as that's my area from an international point of view).
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.
On account of the vast majority (not all, but most) landline area codes being five digits, that is the convention for splitting mobile codes too, so 11 becomes 5,6. Maybe not everyone knows it, but pretty much everyone I know does, from people who spent most of their life pre-mobiles (e.g. my grandmother), to frinds, to young children.
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?
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".
London is one of the few exceptions, not the rule. And even with the three digits, most Londonders I know split their phone number differently (for example "0208 XXX XXXX").
France does not have area codes, just so you know. The second digit (French phone numbers have 10 digits, and the first one has to be 0) is kinda-sorta related to a broad geographical area for some (1 is Ile de France, 2 is the north-west, 3 is the north-east, 4 is the south-east and 5 is the south-west), but not all (6 and 7 are mobile phones, 8 is free and shared-cost services and 9 is for VOIP and stuff which does not fit in the previous numbers)
edit: and the grouping is as simple as you can do: digits are written in 5 groups of 2 digits.
From memory a 0 at the front of a phone number is a stand in for the + prefixed country code. So for example Australian numbers such as 0412 345 678 are actually +61 412 345 678. Same goes for international numbers.
As an 'area code' it now makes a lot less sense, but we still need that many numbers to be able to have 1 for everyone, so it'll still be that long, no matter what it's called.
