Where I’m at now is that I can get VoLTE to register for a “session” (being up until the phone is restarted from what I can tell - testing of sessions at this stage is only for minutes at a time).
After losing VoLTE, I can fix it by flashing a modem sin file using newflasher.
The weird part is that I can’t just reflash the same modem file each time, I end up having to alternating between the XQ-BT52_Customized_SEA_62.1.A.0.675 modem and the XQ-BT44_MVNO_JP_62.1.A.1.431 modem, like it’s allowing a proper settings reset.
Don’t ask me why this works, I really don’t know. This comes from trial and error.
The idea came from flashing Android to my phone, finding that IMS wasn’t registering one time (after flashing the same firmware so many times), completing a factory reset, and then finding that IMS registered without me needing to do anything further.
I’m really hoping someone smarter than me can tell us what’s happening and can get VoLTE going for NZ. I don’t think I can go any further with this as the next step I was going to try would be to explore what’s in the modem files and see if I can figure out what step in the flashing process is triggering a reset of the network information to resolve the issue - but this goes well about my understanding of the world around me.
Edits: realised I made some mistakes in my notes