I used s8 with oreo and bluetooth speaker. I simply put spotify on and set phone to vibrate only.
I'd get back to phone and see any notifications but the music wouldn't be interrupted.
Now I have s10+ and android pie.
Same speaker and method as above but the spotify music gets muted (doesn't ring through speaker just interrupts music).
Then when person cancels call or I do music resumes.
I've tried everything I can think of including turning off "calls" under the BT menu etc.
Driving me insane!