Have you tried talking to the mobile chat team? they can check to see if the sim is enabled now?
Just confirm you have checked the following...
iphone is 6 and above, and now on 11.3
In the phone settings you have now enabled wifi calling
In the mobile data - data options - enable 4G - voice and data is selected
I had a problem with my grandad sim wifi calling wasn't turn on or 4g calling
As a rule BT enable the sim for 4G calling at the same time as they enable it for wifi calling. But depending how we have set up iphones either some of the iPhone dont pick up the update straight away, doing the usual switching it on and off... check the settings are correct..normally does the trick
Not all sims have had the update yet, migrating all the sims will take a while longer yet so its a case of checking the phone then sim
I Phoned up BT they said there a problem with the sim and we have to sent you a new sim as you can't across Wifi calling