In Romania we have 2 carriers which support Volte, oneplus software dont have settings for it , i make to works for me like this

its not my credit , i just found it on a german forum and it works like a charm :
https://android-hubo.de/thread/8127...te-vowifi-freischalten-de-ohne-root/?pageNo=1
1. MAKE a backup of the EFS partition over TWRP or ADB, and put it on safe place on computer or a cloud etc.
2. The Magisk module, must be installed: VoLTE / VoWiFi - Magisk modules
3. Install on your computer
Qualcomm QPST Diag x64 driver
QPST_2.7.460
4. Now you connect your mobile phone to the computer via a "USB cable"
5. Open the phone app and enter this code *#801#
6. Now a new menu appears and switches on "Serial" and "Full-port switch"
7. Start the program: "PDC" and connect it to your device
8. Right-click on "Oversea-Commercial_DS" under "Deactivate" and select "Sub0" and "Sub1"
9. Now press again on "Oversea-Commercial_DS" and below on Remove
10. Now select "India-VoLTE-Vodafone", right-click and select "SetSelectedConfig" "Sub0" and "Sub1"
11. Now press again on " India-VoLTE-Vodafone " and below on Activate
12. After you have done everything, restart the device!