"¿Ustedes no llaman antes?"
I'm not an English native speaker, but I think translating this sentence to the past tense (You did not call before?) would change its meaning here (I think the present tense says something about the general case, and the past about a specific case). Example with present tense: Person A: I always need to wait for hours when I am going to the doctor. Person B: Don't you call before(hand) [...to get an appointment]?