Not quite....
This adds the ability for my iPad (which is on o2 as it happens) to be able to make and receive calls to/from my EE number, even when my iPhone is not on, not connected, not on the same network.
So if you say had a BT Landline and called my mobile number, and I had run out of battery on my iPhone, but my iPad was still good, if it had connectivity (cellular - not needing to be EE... or wifi), my iPad would ring and I could take the call. For example on Saturday my iPhone was at home, I was 15 miles away and my iPad let me take a call from a non-facetime user.
You could previously do this but only if the iPhone was connected/nearby the iPad - so the scenario I describe didn't work.
However, a few clarifications...
EE web site suggests this works for texts - so far, it does not work independently of the iPhone - you could already, on watch and iPad send an SMS but it would relay via iCloud and send to the iPhone to send for you - likewise receiving it. This is STILL the case now - but it's implied in various places by EE that won't be the case - but neither my watch (cellular or wifi) or my iPad can relay or receive SMS if my iPhone is not on and connected - so nothing has changed in SMS terms[1]
[1] not to be confused with imessage which does work regardless but obviously is only of use if the other party is also an imessage user.