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?
07920 748555
0792 0748555
079 20748555
079207 48555