One of the biggest assets mobile network operators have is the ability to handover voice calls seamlessly between different radio access technologies. If the calls starts in a UMTS network and the user leaves the coverage area of the 3G network, the call is handed over to the GSM network without the user noticing it. The reverse direction from 2G to 3G is also standardized but I have yet to observe any network actually doing it during an ongoing call. It could actually be quite useful so background data transfers such as email reception, facebook updates, etc. can continue during a voice call, something that is usually not possible unless the network supports GSM DTM (Dual Transfer Mode).
Anyway, things get a bit complicated in the LTE domain. First, there is no single Voice over LTE solution clearly dominating today, probably due to the lack of networks and devices. But let's say, for arguments sake, that we'll get there one day. How do you hand over a Voice over IP call to GSM once the user runs out of the LTE coverage area. The solution is Single Radio Voice Call Continuity, or SR-VCC for short, which has been specified some time ago in 3GPP TS 23.216. The procedure is voice technology agnostic so it can be used by IMS, VoLTE (an IMS implementation) and also VoLGA.
But once you are there, how do you get back to LTE or UTMS during the call? Well, not at all so far. However, there's a study item currently worked on in 3GPP Release 10 in TR 23.885 that gives a number of options of how reverse SR-VCC could be implemented. How interesting! I'm tempted to read it but I guess I'll just hold off until I see the first (forward) SR-VCC implementation used in a real the network. Have fun putting it into a TS until then :-)