Best network for unlocked handset with vanilla firmware?

[Chas Newport]

I'm on O2 but bought my Xperia outright from elsewhere. Wi-fi calling and 4G calling don't work. They say it's because I don't have O2 firmware.

If I switch networks I need one with native (to standard Android firmware) support for wi-fi and 4G calling. I'm in the UK.

What are my options?

 

[belodion]

Welcome.

So far as I know, you would have no problem on any of the three other UK networks.
You may also be able to find the O2 firmware on Sony’s site.