I want to add multi room to my sub but do I have to get another box or can I just ring them up now and get the thompson box I have activated over the phone and just pay the extra £10 per month?
You will need a second card for the second box. You can order multiroom and the card will arrive within a few days. If you already have a second subscription card that is registered to you (your address) but just needs switching back on, then yes it can be done almost instantly.
I rang sky and they said they would need to send a new card out and a new 12 month sub will start of that box and it must be connected to a phone line. If I want a new box and Sky+ they said I have to pay £109 or something like that!
Shall I just get the new card? There is a old card in the box but they wont activate it as its probably 10+ years old
All multiroom boxes will require to be connected to the phoneline, thats pretty standard but Sky do tend to do more phone checks on boxes which are registered for multiscreen in the way you propose.