What is it with people?
Phone numbers are grouped certain ways because they are. Don’t ask questions. Just obey the rules.
For mobile numbers, they’re structured such: 07123 456789. London numbers: 020 7123 4567. And for more traditional provincial towns like Halifax: 01422 123456. Sheffield: 0114 123 4567. And Freefone numbers: 0800 123 4567.
With the exception of mobiles, the first bit is obvious. It’s the bit of the number that you don’t need if you’re dialling locally. Hence why the 7 is part of the second grouping, not the first. As for the rest, it’s merely convention. Convention that’s not open to debate. OK?
Just because your number may look prettier and more memorable grouped 0712 34 567 89, don’t do it. It grates, and looks ugly. Just as you wouldn’t change the grouping of your postcode.
And if you continue to do so, you’re comparable to those twunts who regroup their car registration numbers in a vain attempt to form something that, if you squint heavily, loosely resembles their name, with a couple of letters missing. More on them another time.