Voice Over LTE is a collective term for any technology that enables voice call from LTE network. So everybody would mean a different specific technology when they say "Voice Over LTE".
I am listing up several technologies which are refered to most commonly.
i) CS FallBack
ii) SV LTE (Simultaneous Voice LTE)
iii) VolGA (Voice Over LTE via GAN)
iv) VoLTE over IMS
Then your question would be which technology should we use ?
There is no solid answer to this question, but one thing for sure would be that the final goal in LTE would be to implement/deploy "VoLTE over IMS".
Are there any network operator who already deployed VoLTE over IMS as of now (June 2012) ? As far as I know, NO. (Please let me know if you see any operator who already started serving this).
As far as I know as of now (June 2012), one of the system operator who is based on C2K network and recently deployed LTE is using SVLTE and most of WCDMA based network operator who just started LTE service are using CS Fallback.