They don't seem to be requiring that for those moved in due to them taking the customers over from Be/O2. The question though is how easy it will be to change underlying phone provider when Sky are essentially providing a different service for the ex O2/Be customers. I suspect though that it ought to be OK - but issues do happen and it could result in no BB.
I thought that Sky would honour existing contracts. Changes to Line provider may annul that.
AFAIK Sky didn't require O2
customers to move from their current line and calls provider at the time of the broadband moving to the Sky network.
The OP would need to look at the terms and conditions to see what would occur when changing from that original provider to another one other than Sky.
I would imagine that a query call to Sky CS would possibly result in that the OP would have to go the route of full LLU i.e. MPF .......
of course I could be completely wrong in that assumption