Easily, if I move to a new house I either:
1) Stay within the same area and hence can move my landline number to the new home. In this case the address is the only thing used and my number is marked to take over whatever number is already on that line.
2) I move to a new area, I cannot move my landline number. The address is the only thing used and a new number is assigned, this takes over whatever number is already on that line.
You generally place your order before you move in, as it takes a few weeks to setup, and also no provider is going to validate for you someone’s existing phone number matches X information without access to an account.
When you move home all you have to go on is the address. The previous person who lived there has their own landline which is their own account. They will likely migrate this to their new address themselves, you wouldn’t take over and keep their landline number.
Providers will therefore just take your new address and move existing / issue new number. All it takes is a digit issue or like in one of my first flats in London, the database is confusing as hell and you have no choice but to take a total guess.
I lived at 105C Uxbridge Road, and the database came up with this (and still does it seems):
https://ibb.co/kD8Rxsd
Well, the flat was top floor, so do you choose
105 Top Fl, Uxbridge Road
The flats also the third floor, (assuming ground is level 1), so Flat Third Floor, also applies.
But technically, I was at 105C, the only flat with C mentioned says 105B - 105C Uxbridge Road, they put it through on this one initially and it didn't go live. All I can hope is it didn't turn them off in B.