I left out requirement for WiFi calling (if a phone is to be 'daily driver'), in previous posts.
I use WiFi more than cellular (basement office & workshop, in rural area = terrible cell reception).
I read that VoLTE functionality shares code with WiFi calling, so the VoLTE functionality is required.
Also looked into Linphone (failed to install native, due to 64 bit not available, also could not find a version to install via Libertine).
The UT old revision device pages for Nord N10 & Volla Phone do NOT mention WiFi calling in the main chart of Supported Features, and also NOT listed as 'Unsupported' below that - just plain missing !
On the bright side, wireless external monitor works OK on the Pixel 3A XL, and is listed as functional on the Nord N10, but as NOT functional on the Volla. (at least on old releases page).
So, if trying for a portable office, Nord N10 appears to be the better choice !
Maybe time to create a spreadsheet comparing status of features across UT supported models...
Does anyone know if a summary (all devices) status chart exists ?