Carriers with wifi calling already manage to transfer calls between wifi and the cell network. However, maintaining network connections between the two is impressive, and quite useful; that makes it much more reasonable to set up various wifi networks and use them seamlessly.
This is a decent article on some of the underlying standards: http://www.radio-electronics.com/articles/cellular-telecoms/... I worked on an implementation of an IPSEC VPN using SIM auth for IMS. GSM standards are amazing in how they make everything work together across many different generations.
