You should be able to contact Sky and ask to be put onto a new offer that includes the phone, at this point sky should then request migration of the phone to there network.
However if they don't budge then it may be worth asking BT what they could offer.
Also the costs for Sky's products is as follows...
Broadband Unlimited (ADSL) £7.50 (Soon increasing to £10)
Sky Talk Weekends + Line Rental £16.40
Non Sky Talk Subscriber Surcharge £5.00
Edited by mlmclaren (Mon 07-Sep-15 14:49:33)