VoLTE is prioritized, so calling shouldn't be an issue once that goes live.
If your devices aren't connecting to the Magic Box's B41 signal, something is wrong. There shouldn't be a need to broadcast more bands from the MB, you either have a Magic Box issue or a device issue. I'd recommend calling support on that.
In any case, if the Magic Box is getting a crap signal from the Macro site, then it can only broadcast crap out of it. Nothing that can really be done until your area has more site capacity. If you get a new Magic Box, you'll soon have the ability to connect it to your home internet for backhaul, rather than the slow cell site, so that may be worth looking into.