If you pay BT for ‘line rental and calls’ and have a separate bill for your broadband from Sky then you must be on a legacy product called SMPF , shared metallic path facility, this is a very old product and hasn’t been sold for many years…..
Sky will have , at some point in the past , asked you to join Sky for line as well as broadband but presumably you didn’t agree hence you remaining on a legacy ( and uncompetitive from a price point of view ) product.
Your SMPF ( Sky broadband /BT telephone) connection isn’t compatible with BT Digital Voice ( DV) as BTDV effectively needs BT broadband , because of this incompatibility, you will be amongst the last to have to change , but eventually your choice will be to either join BT for both telephone and broadband, ( that way DV will work ) , or you join Sky for telephony ( and give up the BT telephone ) what you won’t be able to do is have BTDV over Sky broadband.
