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If you parents have only signed up to Sky, its likely that they are still in contract for that service. If the move back to BT they could be charged a cancellation fee.
If they are happy to move back to BT, I'll be able to take a look to see if their number is available. You can send me in their details using the "Contact The Mods" link found in my profile.
The first stage would be for them to ask BT to take over their service, provided they have not already entered into a binding contract with another provider, as they may have to pay cancellation charges.
If they have only just transferred, then you normally have 14 days to change their mind, and the loss of the phone number, would be a good excuse.
Edit:- Answered by moderator