OnePlus have released an update t their OnePlus 2 in China, that enables the VoLTE on the device. The update comes as Hydrogen OS 2.5, the OS that is being maintained only in China now, as in other regions, OnePlus has substituted that with their OxygenOS OS, something we see on OnePlus 3T and OnePlus 3 too.
The update is available for download, but if you are outside China, we’d urge you to wait for the update to hit OxygenOS OS build. And that should happen soon, now that the VoLTE update is out in China.
If you are okay with Hydrogen OS though, find the download link for the OTA update here.
Also read: OnePlus 2 Nougat release
BTW, the model file, flashable in TWRP, is also available, extracted from the Hydrogen OS 2.5, which can be flashed on a CM14 or any other AOSP ROM, to get the VoLTE service enabled right away. Find it here.
Thanks jagannn and sanchitd5!