The smoothness of this (if it is substantially better than existing VoLTE+Wifi implementations) more than likely has to do with improvements in the handset Wifi implementation and the underlying network evaluation criteria, etc.
Put simply, handsets that are capable of this (and there are many) typically establish an IPSEC VPN back to the IMS network core. When the IMS network needs to switch between cell and IP+IPSEC (usually over Wifi) it is no different than the handoffs that have been happening between individual cell towers since the beginning of cellular. To many aspects of the network the IPSEC VPN looks just like another cell.
The tricky part here is getting the timing right - this is the real reason cell providers and sites have such precise and well coordinated time.
I don't have any experience with it and I haven't seen it work but more than likely the handoff between VoLTE capable carriers is accomplished using similar methods.
Put simply, handsets that are capable of this (and there are many) typically establish an IPSEC VPN back to the IMS network core. When the IMS network needs to switch between cell and IP+IPSEC (usually over Wifi) it is no different than the handoffs that have been happening between individual cell towers since the beginning of cellular. To many aspects of the network the IPSEC VPN looks just like another cell.
The tricky part here is getting the timing right - this is the real reason cell providers and sites have such precise and well coordinated time.
I don't have any experience with it and I haven't seen it work but more than likely the handoff between VoLTE capable carriers is accomplished using similar methods.