@AppLee Thank you for the clear description of the issue. I had a similar problem with my Huawei Mate 20 pro running android - EMUI update v12 never came to Australia due to politics but is required for VoLTE. Once updated it works fine.

I realised during this process that I needed a newer base version of android to enable this feature on the pixel 3a XL. I would be happy to assist in trialing a solution. I am familiar with basic porting process and have a samsung S7 successfully running Lineage OS. But by far my favourite is UB Touch so I'm motivated to find a solution.

Any hints on where I should start? Yes, have donated to the UB Touch project already and happy to contribute further.

Thanks again. Look forward to helping however I can.