That will only happen if Sprint makes it available for all makes and models of phones.
Please excuse the snarkyness of the statement, but I had an HTC M8 that had WiFi Calling, and now I have a Nexus 6 that doesn't.
To my knowledge, the Nexus 6 is capable of WiFi Calling, but Sprint just doesn't seem to want to roll-out the feature for the Nexus 6.