docs: add carrier system apps blocking IMS registration troubleshoot

docs/troubleshooting.en.md

@@ -20,3 +20,41 @@ Resetting network related configurations might help resoving the issue.
 ## Setting APN as `ims`
 
 Some carriers (e.g. Polish Play) require IMS APN to be set as `ims`. Check [here](https://github.com/kyujin-cho/pixel-volte-patch/issues/136#issuecomment-1565598716) for more informations.
+
+## Carrier system apps sabotage IMS registration
+
+This issue is usually the final problem if all solution applied but your Pixel can't successfully register with your carrier's IMS service.
+
+To determine if your phone falls into this category (assuming you've attempted all the suggested solutions), perform an IMEI check on the [Pixel repair check](https://store.google.com/us/repair?hl=en-US) page.
+
+If you see "Unlocked" text below your phone's name, proceed to the next step. Otherwise (usually Fi) ignore this whole section.
+
+1. Removing the Original Carrier System Apps
+   1. Connect your phone to one of your computer with ADB as usual, open `adb shell`.
+   2. List the carrier system apps: `pm list packages | grep verizon`. This command will generate a list similar to the following:
+      ```
+      package:com.verizon.mips.services
+      package:com.verizon.services
+      package:com.verizon.llkagent
+      ```
+      Note: This tutorial assumes the original carrier is Verizon, If that's not the case, try using Sprint (`sprint`), T-Mobile (`tmobile`), etc. or manually browse through the list by `pm list packages`.
+   3. Disable those apps in the system user space: `pm uninstall -k --user 0 <package id after 'package:'>`. For example:
+      ```
+      pm uninstall -k --user 0 com.verizon.mips.services
+      pm uninstall -k --user 0 com.verizon.services
+      pm uninstall -k --user 0 com.verizon.llkagent
+      ```
+   4. (Optional) You can try rebooting your phone now to see if it works immediately, but it usually doesn't.
+2. Restart the IMS registration
+   1. Start Pixel IMS app. Navigate to desired SIM card page and click "Reset all settings".
+   2. Enable VoLTE (and any other desired features) as usual.
+   3. If possible, go to your SIM account on your carrier's website or mobile app and re-register for the VoLTE service.
+   4. Reboot until it works (typically twice should be sufficient).
+
+### Explanation (the boring part)
+
+There is a possibility that your phone was carrier-locked, especially if it was originally purchased in the United States. Those can be unlocked after about two years, depending on the terms of the contract between the original owner and the carrier.
+
+Although after your Pixel is unlocked, and you can use your SIM from different carrier (especially in countries where Pixels are not officially available), those preloaded apps from the original carrier are still persist and manipulate the system to register some special services like VoLTE, VoWifi and 5G data to the original carrier configurations. This can lead to difficulties in registering these services on your Pixel.
+
+You may need to follow these steps again if you ever perform a factory reset on your phone. It's worth noting that some Android 14 QPR1 Beta users have reported that Pixel IMS is no longer needed. This is however possibly removed in the beta ROM for the sake of platform compability, not because Google decided to open their hands.