Hi rosiew
the line is owned by BT Wholesale, and is rented to you through BT retail. Currently you subscribe to CPS, carrier pre select, which means most of your calls are billed by TalkTalk, some calls are still charged by BT, ie when you use 1471 then press 3 to ring the person back. This results in you recieving at least 2 bills, one from BT retail one from TalkTalk
If you decide to have your linerental with TalkTalk too, they you will recieve one bill, and you can only have services on that line which TalkTalk resell from BT Wholesale.
RedCare, is a BT branded service as BT Redcare. I would expect that if your ADT alarm uses Radcare, that this is not comatible with any phone service other than BT.
It is true that the phone line is not altered in anyway, no physical changes are made, no engineer visits your property, the changes are made at the local exchange. This does affect the services that are available on the line, so if this is still related to your ADT alarm you need to clarify if your alarm uses RedCare or a linedialer.