Why are Wifi Calling and iPhone Cellular Calls Mutually Exclusive? I tried enabling "iPhone cellular calls" on my iPhone 5S, and the phone informed me that Wifi calling must be disabled to enable this feature. So it seems that the two features are not compatible with one another. Is this iPhone 5S specific? This is quite disappointing as I was hoping to have both enabled (cell service in my house isn't the best). It would be nice to know why they're mutually exclusive.
In the past, the modem software for a specific carrier was responsible for that prompt as their networks weren't able to re-reount the calls on the fly. (i.e. your carrier or carrier settings needing an update was the original issue)

I believe that window of limitation is resolved for all devices and all carriers. If you get this error in 2017, I would work with Apple support to make sure your iOS is updated and then work with carrier support to make sure your phone account is set up for seamless switch between carrier and wifi calling.