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Sorry to hear that your landline number has not been ported. If you let Sky know that you wanted to port the number, then they should have left the number active for us to be able to request this, if the number is not active we cannot request it and so cannot port it over to your new service.
When your landline service becomes active, you will have a temporary number to begin with, as we'll need an active landline to port the number onto the service.
You are right, that your number will be somewhere, what happens is when a number is deactivated from a service it returns to whoever owed the number originally and goes back into the pool of numbers to be handed to new customers.
Did you port the number to Sky when you joined them or did you get the number from them at the beginning?