[OnePlus 8 Pro Questions & Answers] happiboi:
Well this is awesome! Thanks for everyone's research - I now have 5G + VoLTE + VoWiFi all working on my OnePlus 8 Pro (IN2020 model) on Android 11 (OOS 11.0.2.2) on the Telstra network.

For some weird reason though, the "Telefonica_UK_Commercial" profile via PDC wouldn't give me 5G. Upon further investigation, I found that this profile + the carrier_policy.xml - didn't have the right config for my IN2020 phone.

Subsequently modified the carrier_policy.xml file further and BOOM! 5G is back. Researching the carrier_policy XML further - instead of specific band enabling per the previous config - I edit it to be 'hardware' so that whatever the hardware supports, it will be made available.

Anyhoo, long story short it's all working again thanks to you wonderful people :)

Summary of steps taken:

Prerequisites:
* Need ADB suite available on PC & enabled "ADB debugging" on phone via developer options
* When you permit your PC adb access to the phone after first adb command, make sure you tick always permit this device. When the phone boots after "adb reboot ftm", the screen is inactive so you can't authorize the device when you try to adb shell and are stuck
* Need QPST suite of apps installed on PC (needed PDC tools + EFS explorer)
* If on custom firmware - probably a good idea to backup your phone before you do this. On custom firmware after the first "adb reboot ftm" rebooting again puts the device in a boot loop until you format data
* You'll also need to ensure the toggles are enabled for VoLTE/VoWiFi before heading down this path too:
- Enable the VoLTE/VoWiFi toggles first:
- Download the OnePlus Log Kit app’s APK from > https://downloadmirror.co/1MW9/com.oem.oemlogkit_1.0-1.apk
- Install the OnePlus Log Kit APK
- Open dialer, type in *#800# (If this doesn't do anything, just google search "Oneplus engineer mode apk", download and install it. Then retry)
- Click 'Enter' on the warning
- Select 'oneplus Logkit'
- Scroll down & select 'Function Switch'
- tick the VoLTE & VoWiFi switch options
- Then reboot.
- when your phone comes back after restart, in Settings > Wifi & network > SIM & network > SIM1 (and SIM2) > you should now see the 'Enhanced communications' area have the option to toggle on VoLTE/VoWiFi

Then continue onto the main process:
- Open command prompt on PC & navigate to where your ADB suite is located

- Type command without "":
"adb devices" (I like to do this to ensure adb can speak with phone/computer)

- Should display:
List of devices attached
<if working, your device should be listed here (else, it'll be blank)>

- Type command without "":
"adb reboot ftm"
--> This puts phone into field test mode

- Type command without "":
"adb shell"
--> Creates dialogue for shell command on phone

- In the shell (OnePlus8Pro:/ $ ) type command without "":
"setprop sys.usb.config diag,diag_mdm,qdss,qdss_mdm,serial_cdev,dpl,rmnet,adb"
--> This opens up ports that we used to be able to in Android 10

- Phone Screen is still blank - but it's OK.

- Start up PDC Tool in Admin mode on PC

- Click drop down, select phone connection (there's only 1 in the list)
--> ** If the drop-down box is empty and no phone/device listed, you cannot proceed.
--> ** Other users have foudn this to be related to a Qualcomm driver issue.
--> ** Delete the drivers you have currently installed and get a new one from here:
--> ** https://gsmusbdriver.com/install-qualcomm-usb-driver.
--> ** Also once the device shows up in PDC don't click on it, open efs explorer and let it start searching directories --> ** before going back to PDC and clicking on the device in the drop down.
--> ** The window may say 'not responding' just leave it until the carrier list populates then carry on with below steps

- Do the profile activation stuff for Telefonica_UK_Commercial:
- Look for "Telefonica_UK_Commercial", right click and SetSelectedConfig for both Sub0 and Sub1
- Click on the above profile again and click on Activate
- Do not restart your phone

- Don't close PDC yet (don't know if it make a difference but I didn't close)

- Open EFS Explorer (I opened in Admin mode too)

- Navigate to "\nv\item_files\ims\"
- Replace ims_user_agent file with one supplied
--> The supplied one has Telstra VoLTE Qualcomm as header instead of Telefonica

- Navigate to "\policyman"
- Replace carrier_policy.xml & carrier_policy.xml_Subscription01 with ones supplied.
--> Just take carrier_policy.xml and make a copy named "carrier_policy.xml_Subscription01"
--> ** Caveat, I further modified the carrier_policy.xml file as it wasn't enabling 5G for me on my IN2020 (Android 11) Oneplus 8 Pro.
--> ** I changed the nr5g_sa_bands base & nr5g_nsa_bands base values to be "hardware" rather than specifically band n78.
--> ** This changes config to allow whatever 5G bands the hardware has available rather than specific bands
--> ** You can't enable bands that the hardware doesn't support anyway so makes sense to enable all
--> ** This gave me the needed 5G access with Telstra

- Type command without "":
"adb reboot"

After phone boots back up, 5G + VoLTE + VoWiFi are all intact and in working order!

Again, thank you to everyone's research and efforts to get this all working!

@Mattycee have you given this a go yet ?