Most learners choose between comí and comía by asking whether the action finished. But everything in the past finished. That question can't help you. Ask this instead: is it the event, or the scenery around it? Estaba comiendo cuando llamaste. (I was eating when you called.) The eating was already going on. It's the backdrop, so it takes the imperfect. Your call is what actually happens in the story, so it takes the preterite.