Do we know that the Librem 5 will not support VoLTE, or is that an assumption?
If the modem/baseband module supports VoLTE, then it might come “for free”. (As mentioned by @tg_gpm in another thread.)
The PLS8 module supports VoLTE, but I can’t find a good datasheet that says whether the BM818 module supports it. I’m hopeful that it does.
It might not have occurred to Purism that people would be looking for a 4G icon in their demonstrations. If they knew people were looking for a 4G icon and wanted to demonstrate it, they would still have to find a physical location where the cellular network tends to put calls through on 4G.
Even if a phone supports calling over 4G, the network doesn’t necessarily prioritise its use. Where 2G and 3G remain available, networks tend to push voice calls onto them, because it’s an efficient way to use the spectrum until they are able to get rid of 2G and 3G and reallocate the spectrum to 4G and 5G.
I don’t think you can safely assume that the older technologies will use less power. Depending on the local conditions and how much you’re using the data connection, the opposite may be true. Though that doesn’t overturn your basic point: set it to whichever one is the least power-consuming.
It’s a mistake to think that 5G implies high frequencies. In the short to medium term, yes, 5G will be mostly deployed at high frequencies, because those are the bands that aren’t already being used for the older standards. They are the low-hanging fruit.
But in the long term, as more people get 5G-capable phones, network operators will be able to reallocate an increasing proportion of their lower frequency spectrum for use with 5G instead of for 2G, 3G and 4G. When that happens, people will start to experience longer-range 5G signals.