[OnePlus 5T  Guides, News, & Discussion] poji94:
Hi there, I managed to enable VoLTE and VoWiFi using EFS Tools instead of PDC because I found that PDC does not support Android 10 stock OOS and the changes made is not persistent (SIM changed it will revert back). Previously I am using DerpFest A12 and need to revert back to stock OOS because the OnePlus LogKit only works at OOS IIRC. But don't worry, I will show the way on how to get that working on custom ROM (I'm currenlty using DerpFest A13 - screenshot attached). It's quite lengthy but bear with me..

PC

1 - Install OnePlus USB Driver

2 - Connect phone to PC with ADB turned on in developer settings

3 - Backup required data

- using ADB pull is more reliable than MTP (check for latest ADB version: https://developer.android.com/studio/releases/platform-tools)

- adb pull </directory from phone> </directory of PC>

4 - Copy stock rom zip file (with deleted compability.zip) into phone

- adb push --sync -Z <stock rom file> </directory from phone>

Phone

5 - Flash back to Stock ROM (Oxygen OS 10.0.1) using TWRP recovery. Recovery may be revert back to stock. Flash back into custom recovery through fastboot.

- Finish the setup, enable USB debugging at developer settings

PC

6 - Install Qualcomm Driver (QUD.WIN.1.1 Installer-10037.3 Setup.zip)

7 - Download OnePlus Logkit (com.oem.oemlogkit_1.0-1_minAPI29(nodpi).apk)

8 - Locate ADB Folder

- adb install "Oneplus logkit apk file"

Phone

9 - Open Dialer

- Type *#800#

- Go to OnePlus Logkit -> Function Switch -> Tick

- VoLTE switch

- VoWiFi switch

10 - Reboot device and check settings for VoLTE and WiFi Calling (VoWiFi)

PC

9 - Download, extract & install:

- .Net Runtime 5 (https://dotnet.microsoft.com/en-us/download/dotnet/5.0)

- EFS Tools 1.4

- Nokia 8 mbn files (https://xdaforums.com/t/guide-qxdm-port-activation-on-pixel-2-xl-volte-wowifi-ca.3884967/) <- this device contains more VoLTE supported carrier than Pixel 2/XL

- Find a copy of mcfg_sw.mbn file based matched with the carrier used to the EFS Tools directory. If not found the exact carrier, select the carrier in same country. (overwrite if needed)

Phone

9 - Open Dialer

- Type *#801#

- Choose "Full Port"

PC

10 - Open CMD

- cd to EFS Tools directory

- run command

- EfsTools efsInfo <- check whether EfsTools is ready to be used

- EfsTools.exe writeFile -i mcfg_autoselect_by_uim -o /nv/item_files/mcfg/mcfg_autoselect_by_uim

- EfsTools.exe uploadDirectory -i mcfg_sw.mbn -o / -v

* I only running these commands since the other command with '-s' return me errors.

Phone

11 - Reboot phone

12 - Check for VoLTE/VoWiFi status shown

- Test with different carrier, if any, for better compability test

- if shown then enjoy!

- else repeat step 9

Installing custom ROM back

13 - Enter TWRP

- if not installed, use fastboot method

14 - check for partition matched with supported custom ROM

- format to different file system if needed

15 - Clear system / cache / data / dalvik-cache

16 - Install

- Custom ROM files (firmware file not needed to install. It may overwrite EFS file set and have to do again)

- Magisk (if needed)

17 - Finish initial setup and enable ADB in developer settings

18 - Restore backup file using adb

- adb push --sync -Z </directory of PC> </directory from phone>

19 - Setup everything back (apps, settings and config)

20 - ENJOY YOUR PHONE FOR SEVERAL MORE YEARS!