Vodafone UK Eye Summer Launch for Wi-Fi Calling and 4G VoLTE - ISPreview UK
Mobile operator Vodafone has announced that the commercial launch of their new Wi-Fi Calling service and separate Voice over LTE (VoLTE) technology, which allows their new 4G network to carry “crystal clear” voice calls, should both be deployed this coming summer.

At present most mobile operators still use the existing 2G and 3G platforms to carry voice calls, but VoLTE would extend this facility to 4G, which has so far only be used to carry Internet data (Mobile Broadband) traffic. The VoLTE solution was first tested by Vodafone last year (here).

Meanwhile the other new service, Wi-Fi Calling, will allow customers to harness an existing Wi-Fi network in order to create a “voice signal without the need to go through an app“. Apparently this solution will be “particularly suited to customers living in areas where network coverage is limited or where it is difficult to get an indoor signal“.

Jeroen Hoencamp, Vodafone UK CEO, said:

“Our Wi-Fi Calling service will ensure that our customers never miss a call. It is another important step towards our commitment to build the UK’s strongest converged network
Last year Vodafone said that VoLTE could also help to deliver “high quality video calling” and “faster call set-up times“. Apparently it will do all of this without hurting 4G download speeds or sucking excessive battery life. Soon we’ll be able to test those statements.

But Vodafone aren’t the only game in town and EE has its own plans to deploy VoLTE using the 800MHz band.
Comment: Both O2 and Three launch apps to do this last year. EE are, I believe, still to launch their product which will be similar to Vodafone's