For my upcoming course on LTE services at the University of Oxford in April, I've been giving the voice over IP topic some more thought. Unlike UMTS, LTE is a pure IP based network so it doesn't have an inherent circuit switched voice capability. It's a bit like burning the bridges behind you so you can't go back.
From an operator point of view, potential solutions are the IMS (IP Multimedia Subsystem) or reusing the circuit switched MSC architecture over an IP based channel such as promoted by the VOLGA forum, an approach that I think has a great potential! But what are the benefits of network operator VoIP vs. other purely Internet based alternatives such as Skype or about Grand Central / Google Voice, about which Ajit Jaokar has written recently in conjunction with VOLGA? Well, handovers are!
It's handovers because Internet based Voice over IP services will work well while the network can handover a moving user to another LTE or UMTS cell. But as soon as UMTS and LTE coverage runs out and the system is forced to go to the GSM/GPRS/EDGE network that voice call is history.
Not so with network operator based voice systems. For both IMS and VOLGA, methods are in place to prepare a circuit switched channel in the GSM network before the handover and the mobile device is instructed to use this channel after the handover. In the case of VOLGA, it's a pretty straight forward thing to do. Here, higher layers of the protocol stack will not see a difference between the voice call being transported over an IP data stream over LTE and a circuit switched timeslot in the GSM network.
In my opinion, an invaluable advantage for wireless operators that Internet based voice services will not be able to mimick. And I agree with Ajit, in the future we will see application layer based voice services such as Grand Central and network layer based voice services of wireless operators working together instead of fighting with each other.