As Nintendo points out in the app's official FAQ, voice chat cannot run in the background while using your phone for other purposes such as "texts, social media, etc.":

"The voice chat will disconnect while you're talking on the phone or using another application, but your voice chat will restart in the same room once you open the Nintendo Switch Online application again as long as the game session is still ongoing."

What's more, early testing confirms that the app won't work if your phone goes into power-saving "sleep mode," with the screen inactive. And don't bother trying to use the app to chat with your Switch friends while away from the system, either: you can only join chat rooms with people when you're actively playing a specific, supported game with them.

This is all in sharp contrast to Sony and Microsoft, which have allowed simple party chat directly on their consoles—across games and at the system level—for years.