If it is 100% not doing it on an iPhone7. But 100% does on the new iPhoneX. Then by default, it automatically IS a problem with WiFi calling on the new iPhoneX. And there is nothing you are going to be able to do about it, you are not Apple. You can't create new firmware and push it to the phone.

I would explain what is obvious for the customer. And inform them they are just going to have to wait for a patch that will fix this problem.
There is also a chance that it is something ubnt can fix in a patch from their end. I dislike this approach when manufacture do this because:
1) It makes it seem like the fault was in your equipment, from their eyes.
2) They think Apple can do no wrong and is flawless. Someone else creating a work around to get it to work, only reinforces this stupid ignorance in Apple users. Happens all the time.