Samsung
Reject an Incoming Call auf Samsung
An incoming call on a Samsung phone shows two round icons: green to answer, red to reject. Both work with the same gesture, so mixing up the direction is a common way to accidentally pick up a call you meant to decline.
Reject a full-screen incoming call
When the call fills the whole screen, press and hold the red icon and pull it away from the center. The exact path is Tap and hold the red call icon ➔ Drag the icon outside of the circle. The direction does not matter, only that the icon leaves the ring around it.
Reject a call that pops up over another app
If you are already using another app and a call arrives as a small floating window instead of taking over the screen, dragging is not required. A dismiss option appears directly in that pop-up, and a single tap on it ends the call for the caller without opening the phone app.
Releasing the red icon back inside the ring cancels the reject action, so the phone keeps ringing. If nothing happens after dragging, check that the screen is fully unlocked, since a locked screen can change how the gesture reacts.