I hated this so much on the S10, because it would interrupt games, apps, everything (and my network coverage sometimes flipped from 1 bar to 0, so I was seeing this 20-30 times a minute) but I found a solution that has worked for me so far (I've definitely seen my network transition to Emergency calls only without popping this up). Fair warning, I don't know what other consequences this had, but basically, I went to Settings -> Apps -> triple dot menu -> Special access -> Appear on top -> Show system apps -> (then I went through a bunch of apps that I thought could be the culprit, and turned them off. That included the phone apps). I've tested to make sure that if someone calls me I still get notifications (audio if phone volume isn't vibrate/mute, and full screen popup if I'm in another app). For completeness, I also went through the system apps in Settings-> Apps and turned all their notifications to silent and minimized, but as far as I can tell, that didn't help. I haven't gone back and turned stuff back on, because I never want to have this happen again.