OnePlus 3T recently got updated to Android 7.1.1 Nougat with build software version O2OS 170324. However, few users took to the official OnePlus forum and disclosed that the update has removed VoLTE support for them.
The issue was first reported by madhan3008 and confirmed by other users directly on the forum. However, some users are not facing the problem. Hence, you can assume that the bug has affected only a cross section of users.
If you own OnePlus 3T and affected by the bug, you will not view the button to enable VoLTE from the Settings app. Hence, your handset will not work with 4G VoLTE enabled networks such as Reliance Jio including making phone calls via VoLTE.
Even though the bug was reported on March 30, 2017, the company hasn’t confirmed the presence of the bug. As of writing this, the company not yet acknowledged. There are reports that users in India using Jio 4G are unable to make any VoLTE connectivity after update to version 4.1.3. Moreover, the bug is also present in the latest open beta.
We expect OnePlus to release an update via OTA to address the problem. If you are facing the problem, you should post about it on the official forum. You should check out the official OnePlus forum daily for updates. That said, you will have to wait till the company pushes the next update.