I have the Nokia 7 Plus and according to the BT Mobile App it is not wi-fi calling compatible but when I am in a poor signal area (e.g. at home and at work), all of my incoming calls are identified in the notification area as wi-fi calls
I just think it's poor communication from BT, they offer the incentive but don't tell you how to get it or anything about making it work
It's on your order confirmation mate, which is sent once the order has been placed, at most it takes 24 hours but is usually instantly sent, see image below.
That link that states Sign up for free BT Sport. takes you to www.bt.com/sportonthego which has all you need to order and get it working.