Light Reading has recently posted an interesting article on how much power Voice over LTE calls draw compared to circuit switched calls today from a smartphone battery. In the report, Metrica Wireless is quoted with a power consumption measurement of a VoLTE call of 1358 mW compared to 680 mW of a circuit switched call over a CDMA network.
No further details are available so it's difficult to tell why the difference is quite significant in the device tested. It could be many factors such as a still non-optimized air interface use for voice over IP on both the network and the smartphone side. Also, there's an additional overhead in the smartphone itself for using the baseband processor for the radio transmission and the IP stack and voice compression on the application processor versus the standard circuit switched voice approach in which all processing is done in the baseband processor.
For people that mostly use their smartphones for non-voice related activities will probably not care very much. For those who make a lot of phone calls during the day, however, the difference will be quite notable.
The interesting thing to look out for now is by how much these initial values can be improved over time. And there is still some time left as there are only few VoLTE deployments and users so far and LTE network ubiquity in most countries is still far from GSM and UMTS.