My 10iii was on Spark.
In the end I gave up. The breaking point was being not able to use the phone in Oz. The Aussie telcos blacklist phone IMEIs not within their “approved” range so couldn’t even use 4G. I go there 2 or 3 times a year plus up into the Pacific Islands. I needed something to just work.
I bought a late model Samsung which fixed the VoLTE and roaming issues in Oz.
I’m still interested in getting the 10 iii going on VoLTE. When I have time I’ll try what tn71 has done, providing VoLTE works on Sailfish. Based on my understanding, if it works on Android it should work on Sailfish.
@tn71 where did you download the ROM from?