Ive been told different stories by different customers/service agents.
The most common one is out of contract and threatening to leave, usually attracts a loyalty discount in exchange for another contract term.
The best explanation was from a service agent who told me it was related to the cost to VM of your installation and how much of that they have recouped. That seems attractive as a theory but I doubt its actually that in depth. Most stuff with VM seems bound by the criteria of " who you speak to on the day and what sort of mood they are in"