I completely agree with you, that in normal English, we would say "If you call me, I'll come immediately." As a matter of fact, that is what I put, and it was marked wrong. The suggested translation is literal, but sounds totally unnatural in English. Both answers should be accepted.
I agree with NaftaliFri1 that "calling to someone" in the sense of shouting for their attention is certainly fine if a little uncommon. And something can call to you in the sense of attracting you ("it calls to me").
While you can use the present for repeated or general actions that may be in the future, I agree that it sounds a little odd here and I'd more likely say "if you call to me I'll come immediately." I suppose if I were making a general statement about my usual behavior (as in, "I come if I'm called") it's alright, though.
It's not necessarily unnatural English, at least not with respect to the use of the present: "If you call [to/for] me I come immediately." The present is often used in such expressions to express general, generic, or gnomic ideas (i.e., states of affairs, maxims, proverbs, and general truths that are generally the case). Similar uses are found in English, Hebrew, (Koine) Greek, and no doubt many other languages as well.
The future is yet another tool that English speakers have in their toolbox, although it'll convey its own subtle nuances and conceptualizations. If they had wanted to make a statement that focused more directly on the future, they certainly could have done so. It's not like English and Hebrew are lacking the resources! That's probably just not the focus of this particular lesson. Those lessons come later (in the future!).
Part of the problem might be that DuoLingo often lacks context. That's something that users just have to be aware of and ready to supplement on their own (e.g., by imagining the more typical, or even atypical, situations in which you'd encounter such expressions).
AdamOlean has explained above that the simple present in English means what always or usually happens; and that, as he correctly put it, a Duolingo sentence "lacks context". It gives rise to many fascinating speculations in these comment forums. So, here's a possible scenario that takes account of the oddly worded English sentence to which some of us have objected so strongly. Parent accompanies a nervous child to the playground, where they've been many times before. Parent sits down on the bench at the side, but the kid whines "I need you to be with me when I go on the climbing frame." Parent (actually it's dad -- I derive this from the use of the masculine in the Hebrew) replies reassuringly, "Don't worry son, you know that whenever we come here, if you call to me [from across the playground] I [always] come [to you] immediately." No need to change anything in the English in this particular instance.
This is a conditional sentence, not future. If we were to say "I'll come" that would also need to be in Hebrew - אני אבוא, but it doesn't, so present tense is correct. Besides, at this point, the future tense isn't introduced.
Not sure why you are so upset about a simple exercise.
Another thing. If you do want to get the attention of the course creators, use the report button. Don't write here, since they will most probably not see it here.