"who is called by them?" is a passive sentence, just like the Indonesian sentence, using "di-" verb.
"who did they call?" is an active sentence, in such a case the Indonesian sentence would use a "me-" verb and would have another sentence structure.
The sentence "Who is called by them?" sounds a little awkward in English but the meaning is understood. I heard that this kind of phrase (the passive voice) is much more common in Indonesian, does anyone have any thoughts or tips about that?
Perhaps in cases such this, and I think it is a good learning process, that Duolingo provides the translation in both pasive and active voices, noting that the "corect" translation sought is in the passive voice.