- Voice over LTE support enabled for selected operators.
- Native Wi-Fi calling support enabled for EE UK and T-Mobile USA.
- Additional improvements to stability, audio quality, Wi-Fi and cellular connectivity, national roaming, and more.
Wi-Fi calling provides high quality audio and is only available on certain carriers. In the case of the Lumia 640 and Lumia 640 XL, the only way to get it at the moment is to be on EE in the UK or T-Mobile in the US. The update specifically lists support for T-Mobile USA which is noteworthy because at the moment, the Lumia 640 is not available through that carrier.
In the United States, the Lumia 640 will be available on AT&T, T-Mobile,and Metro PCS, and the Lumia 640XL will be available exclusively through AT&T, at least for now.
Windows Central points out that both unlocked and regional variants of the devices are able to receive the update.Further reading: Lumia 640, Lumia 640 XL, Windows Phone