Wifi calling

My wife has a OnePlus Nord 2 5G with the exact same issue as the OP -  when I contacted SMARTY over the issue about 6 months ago, I was advised that the phone wasn't compatible for WIFI calling on SMARTY and that it MAY be provisioned some time in the future, which to me is a bit of an awful response.  Surely WIFI calling should be available to any and all phones that has the feature built in?  And I don't understand why it would be such a problem when other phones will work that aren't on the compatible list, including my children's cheap Motorola Handsets, my old Samsung S9+ and my new iPhone 12 Pro Max.  This is a big issue for my wife as we do not get a mobile signal unless we are within 3-4 feet from the windows of our home and so she has to rely on WIFI calling to make use of her mobile phone.   Upto now, we have had to use third party services, namely Whatsapp, to make calls to her when she is at home as that is the only way the phone will work without a signal.   As a result, SMARTY are going to lose a customer as we need to move my wifes mobile from the network to one which allows the OnePlus Nord 2T 5G to work with WIFI calling just so she can use the minutes and data that she has been paying for but is unable to use whilst at home (unless she stands outside).