It most likely works in your area but the provisioning has not been done on your specific account. It was the same for me but via chat and a little time I had that fixed. I explained that I wanted Wifi calling and that I had access to the correct hardware and I am in an area with VoLTE which is a current requirement to participate in the trial. Try a chat with AT&T and if they do not follow, simply try again. Its a trial service so maybe not all agents are aware of the details yet.