The OnePlus 2 received a fix for its recent VoLTE support issues in the form of an OTA update to OxygenOS 3.5.9. Aside from the VoLTE fix, this update does not seem to contain much and is listed as being relatively incremental. OnePlus has yet to post an official changelog detailing the package, but it is already available for download from the company's forums. This, of course, means that its rollout to all OnePlus 2 users should be underway by now. For the time being, those who don't want to wait for the OTA or can't get it for some reason can only download the full, signed, and flashable ZIP file which contains the entire gamut of system files, and must be used in TWRP recovery, with the package itself weighing approximately 1.2GB.
For reference, this issue came to light with the OxygenOS 3.5.8 update. The incremental update was meant to update the Android Security bulletin to the March level, fix up a few bugs with cellular networks and Google Duo, along with introducing "general bug fixes." However, many users were surprised to find that their devices could no longer properly utilize VoLTE to make calls following the update. A forum thread about the issue was eventually marked as resolved when users collectively gave up and decided that the best solution would be to simply roll back to the previous update. This software is still based on Android 6.0 Marshmallow and according to OnePlus itself, the device won't be getting an upgrade to Android 7.0 Nougat.
Updates for OnePlus phones hold a few extra precautions compared to regular ones, especially when done manually. For starters, a phone that has root privileges enabled or is running any kind of modification over its stock ROM, such as a custom kernel or modification framework like Xposed, will not be able to take the OTA update, and if it's forced, it may fail to flash, or simply not boot. This means that those wanting to take the update will have to flash back to bare stock first. Users flashing an incremental update and not a full, signed, flashable ZIP file must first flash a signed ZIP from OnePlus, usually the previous update, then flash the new incremental one. Dirty flashing, which is flashing an update without wiping your device, should work fine if you're coming from the previous version of OxygenOS, but not further back, or from custom ROMs or other software. Beyond that, the normal caveats apply; avoid powering off the device during the procedure and ensure there's a good amount of battery charge left or the phone is on a charger while the update is being flashed.