Spotify starts playing out of phone speakers when phone receives phone call or similar

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As title explains I'll be listening to a song on the Spotify desktop app with my phone locked and the Spotify app closed when a triggering event such as a phone call causes the music to stop playing out of my PC headphones and instead blares it out at full volume on my Galaxy S7 device.

This is not something that can happen at work, and I'll be cancelling my Spotify subscription if it isn't solved.

I've searched the Community for answers but as my software is up to date I've been unable to find a solution.

I read a thread that said that each situation would be dealt with case by case, so here we are.