Sky unbundles the broadband and phone service to its own hardware, so the line is still Openreach copper, but the number will not show up as a BT based number on the checkers, because it is not a BT based number.
If you have moved in, then the fact that the old bill payer has moved out should trigger a cease, and you will need to order a new line (which would then be provided usually over the existing openreach copper). It is not a migration at all.
Be is a shared LLU service, so requires you to be paying voice line rental to someone who provides a voice line rental product, e.g. BT Retail, PostOffice, Primus and a raft of others.