South Korean operator SKT and USA’s MetroPCS have both announced their launches of Voice over LTE (VoLTE) services.

SKT will brand its VoLTE offering ‘HD Voice’ and said that the service will allow it to begin cutting the ties with legacy voice networks as well as better compete with over-the-top VoIP services.

The operator claims that VoLTE will dramatically improve audio quality, relying on high quality audio codecs that are able to handle 2.2 times wider frequency bandwidth as 3G voice, and call connection time will also be “up to 20 times quicker” than the five second average for 3G.

Meanwhile, MetroPCS claims VoLTE will improve its spectral efficiency and network capacity. The operator is selling the VoLTE-capable LG Connect 4G Android smartphone at select US stores and will continue to roll out VoLTE services and phones in the coming weeks.

Currently on 4G networks LTE voice calls are supported in the 3G network through an implementation known as Circuit Switched (CS) Fallback in 3GPP networks. An unintended consequence of this standardized approach is that if an LTE subscriber is using an application, such as Facebook on the 4G network at the time of the call, both the voice and data session drop back onto 3G.

“Until this data session is complete, or the device reset, the subscriber is camped on the 3G network, which places unnecessary pressure on the 3G and leaves the subscriber under the false pretence that they’re still using 4G, but not getting the level of service they expect,” said Marc Bensadoun, CEO at network visualisation software provider Newfield Wireless.

Jonathan Bell, VP of product marketing for telecoms software firm OpenCloud added that VoLTE offered operators a chance to defend themselves against OTT VoIP applications. “While OTT providers have capitalised on the growing demand for VoIP, operators are responding through VoLTE investment to defend voice revenues,” he explained.

“VoLTE will provide operators with the opportunity to establish voice and video propositions that the OTT providers simply cannot match – with the provision of voicemail, roaming and interconnect – all the services that subscribers take for granted when using mobile voice.”

But not all operators are convinced that the move to VoLTE is a priority. “We have no clear time frame for VoLTE,” said Christian Daignault, CTO of Hong Kong’s leading mobile operator CSL, which launched LTE late in 2010. “Since we deployed our circuit-switched fallback solution the voice quality, the handover and the call set up have been so good that there’s just no pressure to deploy VoLTE,” he said.