This isnt assured, I didnt get the call, and a fair few on reddit report the same. But if you do get the call it does work though.you dont have to go though with the disconnection, retentions will call you back and give you a better offer, I do the same thing every 18 months, never had to go through with the disconnection and they always offer a price better than the offer new customers!
Avalon on here mentioned it might be related to how the customer is scored internally in terms of potential future revenue, on the likelihood of getting offered a good deal.