Why is a preposition in this sentence needed?
So here's the sentence that I found in my textbook: Du musst noch beim Reisebüro anrufen. Why is that "bei" needed? Can't I say Du musst noch DEM Reisebüro anrufen? If the latter is correct, too, what is the difference?
Not "dem Reisebüro", but "Du musst noch das Reisebüro anrufen" (accusative); but then it's correct, too.
(to call [by phone] = anrufen + accusative; vs. bei + dative)
When you use "bei", it sounds a bit more personal, i.e., more like "I call [a person] at the travel agency", not just "the travel agency".