Nokia E-series handsets can be persuaded to support VoIP over EDGE and it sort of works (few years ago we did some testing of that with intention to use that as part of cab telematics), one would say that it works better (certainly it's more reliable) than VoIP over WiFi with same handsets.
Interprotocol handoff between two CS protocols is mostly equivalent problem to normal GSM handoff, handoff of packet switched transport is mostly transparent to higher level applications (ie. voice). Clear problem with IP->CS handoff is sudden change of QoS parameters of channel which I think cannot be overcame in any sane way and any solution involving IP->CS handofs will reintroduce audio artifacts of NMT. (hypothetical insane way: pump full RTP datagrams through UMTS radio channel instead of bare audio, which is impossible to do with GSM radio layer). CS->IP handoff (which seems to me to be more interesting in czech environment) has same problems and also lacks any support in existing CS protocols that are mostly set in stone.
On the other hand I think that LTE deployment will go reasonably fast so these issues can be mostly ignored by doing fallback to CS where possible and using only LTE when deployment is sufficient. So I don't think that anyone is going to seriously consider solving problems inherent in CS<->IP handoff.
Interprotocol handoff between two CS protocols is mostly equivalent problem to normal GSM handoff, handoff of packet switched transport is mostly transparent to higher level applications (ie. voice). Clear problem with IP->CS handoff is sudden change of QoS parameters of channel which I think cannot be overcame in any sane way and any solution involving IP->CS handofs will reintroduce audio artifacts of NMT. (hypothetical insane way: pump full RTP datagrams through UMTS radio channel instead of bare audio, which is impossible to do with GSM radio layer). CS->IP handoff (which seems to me to be more interesting in czech environment) has same problems and also lacks any support in existing CS protocols that are mostly set in stone.
On the other hand I think that LTE deployment will go reasonably fast so these issues can be mostly ignored by doing fallback to CS where possible and using only LTE when deployment is sufficient. So I don't think that anyone is going to seriously consider solving problems inherent in CS<->IP handoff.