Even tho duo lingo said chi chiama means "who is calling" it literally translates to "Who calls". Now idk if duolingo put the answer as "who is calling" because in italy they might use this as a term to say "who is calling" like a shorter sentence. Idk because im a learner like you but sometimes i guess we gotta research or improvise with duolingo to get the best results at times.
I'm guessing because the present tense can be translated different ways...
I'm a beginner at Italian, but I know that in Latin you can translate any present tense verb like this as "calling", "is calling" or "do call". And as in questions, you use the present as "do call", I'm guessing that "is calling" is always an option for any verb in the present, but we just have not used this much.
"Chiamare" is a verb that doesn't work with the preposition "a", so you would use it like:
Lo chiamo = Chiamo lui / Mario La chiamo = Chiamo lei / Marta
(working with direct pronouns / pronomi diretti)
On the other hand, "telefonare" does work with "a", so it would change things a little:
Gli telefono = Telefono a lui / Mario Le telefono = Telefono a lei / Marta
(working with indirect pronouns / pronomi indiretti)
It all depends on the verb working or not with the preposition "a", and the whole thing of verbi transitivi e intransitivi