I have been using AT&T Wi-Fi calling since it was first available in the iOS 9 public beta. When I am home my iPhone is usually in Wi-Fi because the cell signal is weak (South Florida house with rebar in the walls). I have not had any problems with sending or receiving SMS messages. I do think there may be a problem with NumberSync, however. When I turned it on, I had no problem sending an SMS text, but I know I missed receiving one. After I turned it off, my texting worked properly again.