An assist ball with some other icons appear on phone while on a call.

Madd62

Mr. Wonderful
May 31, 2021
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Is it from Settings - Additional Settings - Quick Ball.

I'm quoting the Settings names from an English Mi 10 and Poco F2 in the UK.

This has a small white curve on the side of the screen (movable) which when tapped shows 5 app icons (selectable) which open that app.

I don't know why it opens during a call except probably it's not very visible and you accidentally touch the white, curve, thereby making it in to a ball surrounded on one side by app icons.

However this feature is mostly made superfluous with a new feature on MIUI 12.5 or 13. If you go to Settings -Special Features - Sidebar a white vertical line draws out in to several vertically arranged app icons and these will open as a floating small window (adjustable) on top of the current screen.
Both features can be turned off if they are annoying, or dragged to a different area of the screen edge.