Outgoing voice setup is all On Sky and can be set up in advance including the CLI ( Same as CLI spoofing it can exist in two places).
Incoming has to be ported through from BT to Sky. So Sky have to build it in their systems then confirm to BT who can then port it. If this is done in the wrong order a call can bounce around in circles causing the appearance of congestion.
In order to prevent this any unused number prefixed by a porting prefix is meant to be given NU tone / announcement. Until all the steps are correctly completed this is what you get.