There is a scheduled date of June 13th now for the migration of people that have already signed up. I am not sure what the cut off is, or will be, for this date. I am hoping that due to the year long delay in us getting service that we might see this date pulled forward but I guess I should just be patient.
When you get the note from Call Flow letting you know that your order is in to Openreach you should notice that your phone service will take a couple of weeks to follow. I am not sure what you will get in terms of communications from other vendors but if you are currently with BT you will get a note from them saying they are sorry to see you go but noting that your phone service will be staying with them. When you see this remember the note from Call Flow!