No it's real VoNR. It sets up the equivalent of a dedicated EUTRA session (I forget what it's called in NR) for it with the proper bearer. NSG isn't really able to decode a lot of the VoNR signaling to get codec details and such yet. There are likely a lot of issues with it still. It's probably not supposed to be public yet, and it's a fluke that you can access it by forcing SA. It probably won't launch more widely until the x60 comes out, which is the first modem that's supposed to support it. Regarding the SIM swap, I doubt it. In my experience, changing the SIM resets all locking back to none/default. But you can certainly try.
Question is whether SA would stick after a SIM swap back to Sprint. I have a Straight Talk data-only SIM that I could reactivate that sits on TMo, so I could test this.
As for VoNR support, betting that it isn't actual VoNR, but basically the NR equivalent of CallingPlus/WiFi calling, where calls run over IP without QoS. Same idea as VoIMS on the iPhone when you don't have service from one of the SIMs.
If you had a T-Mobile SIM you could get into the service menu and force SA. The Sprint SIM locks out the dialer code. It appears that the S20 actually has some support for VoNR when locked to SA on T-Mobile. The signaling (captured via Airscreen, which is basically the PC version of NSG) seems to show everything as NR without LTE fallback, since the phone advertises to the network that it doesn't support LTE when locked. And this seems to only work on the S20, not other SA capable 5G phones. But there's still a little uncertainty with it. Either way, T-mobile is making progress with it and appears to be ahead of a lot of providers. Sent from my Pixel 4 XL using Tapatalk