Well the people at BT say otherwise. I looked at the link you sent. It says nothing about retaining phone numbers.
I have done enough research in the past few days to give me an NVQ and a headache. I would like someone knowledgable at BT to actually listen to me.
This community forum has been of the most help do far - but as jac95 below says, no one has been trained:has sufficient knowledge to assist.