The BT Phones are DEC phones so only the main one needs to be plugged into the mains. Any Extra handsets need only a power point to charge but do not need to be plugged into an telephone extension.
So anything else plugged into an extension phone socket would probably ring.
The Main BT phone prevents any of the Nuisance calls ringing unless the number is on the allow list or the caller has pressed the hash key and said there name.
The BT Nuisance Phone work as a group.
Share contact details and block lists.
They are worth every penny if your getting sales calls.
You can add on a phone at a later date if you need to, but when i was pricing them it was £5- 10 difference for 2 or 3.
Don't buy from BT Store as they say 33% discount but it works out exactly the same price as Argos, so go there and if you don't like them take them back
Hope that helps