I'm not sure that's been stated or demonstrated/proved.
When using any phone provider that uses BT's Wholesale Line Rental, you will be connected to BT equipment at the exchange.
@Discus - Dial 17070. If you are connected to BT equipment it will tell you it's "BT line Test Facilities". If you are connected to Sky (Easynet) equipment you will get a different female voice with a different set of options (don't recall the exact message).
Just for clarity, although it shouldn't make any difference to your ability to have a correctly functioning service - when you joined Sky, you were given a brand new number by Sky?
Whatever the actual cause, and by whatever mechanism the number port should actually work, it is your CP's responsibility (ie Plusnet) to make sure the service works correctly. I would deal with the problem in the following way -
1) You are paying for a Caller Display Service - if this is not working correctly, you are entitled to complain and request a refund for the periods of time that it hasn't worked (correctly). You should check using a totally separate 2nd phone (1st disconnected) and confirm that there isn't a fault with the phone (irrespective of what 1471 says). Tell Plusnet you have carried out that check and there is a fault with the service and request a refund. If Plusnet are being "difficult" about this, tell them you are cancelling Caller Display until such time as the problem is resolved.
2) I would do this anyway, Log into the member centre and cancel the Caller Display Service. (It will be interesting to see if there is any interaction between the Caller Display Service and what is being given on 1471). When it's cancelled, use a different phone (Caller Display phone disconnected) and confirm that 1471 is still reporting the incorrect number. Complain to Plusnet. It is their responsibility to contact Openreach and Easynet/Sky if needed to pursue the issue. It is also Sky's responsibility to ensure that the number port is working correctly.
You are paying line rental for a service of which part is not working correctly. (your inability to return a call should you wish - note even if it is a chargeable service when/if you use it). If Plusnet are failing to pursue this, then threaten to go to OFCOM and do so.
Plusnet customer since June 2003.