This is the reply I got from One Plus support. All true but not very helpful.
Dear Customer
Thank you for contacting OnePlus support.
There are three prerequisites for providing end-to-end VoLTE services.
SIM card - Your phone's SIM card must support the ISIM embedded application.
ISIM stands for IMS Subscriber Identity Module.The ISIM contains the credentials needed
for VoLTE authentication with the network.
Without the ISIM application on the SIM card, even if your phone supports VoLTE, you cannot
use the service.
Handset - Your phone must support VoLTE technology in its software and the underlying modem.
Therefore, if you try to use a SIM card with an ISIM in a handset that does not supports, your service will not work.
Operator's network - Even if you have a SIM card with an ISIM and a VoLTE-compatible handset, your operator's network
must support VoLTE technology . Without proper network implementation VoLTE services will not work.
If you want to enable VOLTE in your phone..... Then there is no manual technique you can perform to turn on VOLTE....
If you use a sim that uses VOLTE, it is automatically enabled....
Actually, VOLTE is a function of your sim and not your phone...... So, if your phone supports LTE it will definitely enable VOLTE
Each mobile operator must negotiate with the phone manufacturer to implement Volte on a device of their choice.
Every OnePlust device supports VoLTE technology.
https://community.oneplus.com/thread?id=1501093