OK that is the expected outcome. HD Voice is a pre-req to enabling Wi-Fi calling.
While I was focused on the iPhone hardware. @
unbound spiral reminded us in 2 posts yesterday that this issue can also be caused by having the wrong Device information on a line. For example if the line incorrectly shows a iPhone 5. Where have we seen that before, a problem that happens every year during the mass upgrade cycles.
You can check myAT&T to see what device is listed for her phone line. Just Click from the header bar the navigation icon Select MyAT&T > My Wireless > Manage my devices & features > See what device is shown for her line.
A call to a Customer Care rep will allow you to update the IMEI to match her iPhone 7+
Note it still could be the actual iPhone 7+ hardware, so if the information above does not fix it we a back to needing the Model information from Setting > General > About.
Dave