My 5C iPhone does not have Wi-Fi Calling under Settings>Phone with IOS 9.3.3

According to the technical specifications for the iPhone 5c, it does not support wi-fi calling. iPhone 5c - Technical Specifications You would also need to be on a cellular provider that supports wi-fi calling, but you would need the iPhone 5s or greater for wi-fi calling.

While technically a device may support the feature, it is up to carriers to support that feature. AT&T limits wifi calling to iPhone 6, 6s and SE models only. You can call them and ask about it, but they simply don't make it available on older iPhones even if the older hardware is compatible with the feature. Their network so their decision what devices they will support the feature with.

Of course the carrier has the ability to affect select carrier-controlled settings via over the air updates and such. They and the settings they've provisioned on your account will, for example, determine if the "Personal HotSpot" setting shows up or not. Similarly for wifi calling. They can choose which settings are present and enabled under the "Phone" section as determined by what they enable on your account.

If AT&T decides not to support a carrier-controlled feature on particular hardware, or on someone's specific account, they can indeed control what features settings appear or are useable on your device.

Apple merely makes the hardware and iOS compatible with a feature, but carriers control which features they choose to support on their network, and for which devices and accounts they wish to provide those features to.

The link I provided earlier does not list wi-fi calling under Audio Calling capabilities. That is why I provided the link. It does appear for the iPhone 5s and above.

Your cell provider does not have wifi calling.

ChrisJ4203 wrote:

According to the technical specifications for the iPhone 5c, it does not support wi-fi calling.

Where does it say this?

The iPhone 5C does support wi-fi calling.

-> Make a call with Wi-Fi Calling - Apple Support