Listening to Spotify via any Bluetooth device (car handsfree, Airpods etc).
Received a call via any 3rd party app (whatsapp, viber, signal etc).
Playback pauses for first few seconds and then resumes with 50 % volume. So the call is combined with music on Bluetooth device.
Not possible to stop playback from lockscreen, only in app.
iPhone X iOS 11.3.1