At the end of last month, Bouygues Telecom announced to become the first French operator to launch the VOLTE in France (read: VoLTE: Bouygues Telecom, 1st operator to offer 4G calls in France).
At the same time as the release ofiOS 9.2 last night (read: iOS 9.2 available on iPhone, iPad & iPod Touch), Bouygues Telecom made available to its subscribers an operator update (stamped 23.1).
With the latter, users can use the fourth generation network for calls (voice) and data, in Settings> Cellular data, and so take advantage of the VOLTE. Previously, they could only activate the 4G only for data.
Unfortunately, it will be necessary to wait until 2016 to be able to really benefit from it, the service being for the moment limited to 5000 privileged subscribers. VoLTE will be available on the iPhone 6, iPhone 6S, iPhone 6 Plus, and iPhone 6S Plus.