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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.

Tried, no go. Said they are waiting for FCC approval to turn it on and only ios 9 beta users could be part of the test still.
 
This is really weird. My 6 Plus enables wifi calling and works great. My wife's 6 Plus give the oops error when we try to enable it. That makes no sense.
 
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