VoLTE calls fail on FP4 with Windtre carrier (not possible to pick up the call)

• /e/ version: 1.9
• Device model(s): FP4
• Developer mode enabled: yes/no
• Device rooted: yes/no
• Trackers blocker enabled: yes/no

Summary

It is not possible to receive an HD call (VoLTE call) on FP4.

The problem

Steps to reproduce

I can swipe to answer the call, the screen changes as if I've actually answered, but the caller is not aware of that. The calling tune still runs until they end the call as if I hadn't answered at all.
Both sides must be using 4G to make the issue happen. (when the "HD" symbol appears on incoming calls)

What is the current behavior?

Cannot perform an VoLTE call (HD call)

What is the expected correct behavior?

VoLTE calls should work

Technical informations

Relevant logs (adb logcat)

Relevant screenshots

Solutions

Workaround

Disabling the 4G enhancement called "4G calling", makes the issue disappear, but then calls are not VoLTE.

Possible fixes