Translation:I will have called her before you have arrived there.
It is there, but I think it's hard to hear the "-e" of "henne" before the "i-" of "innan". To me it sounds a bit like "...ringt hen-innan..."
My best advice is just keep practising. I find listening the hardest aspect of a language, but I can definitely feel progress with Duo, over time :-) Things that once sounded impossible now sound like clear words, especially with longer sentences where I used to just get lost!
It isn't really, it's the same as the other sentences in this section. If you consider "Du kommer att ha kommit dit" / "You will have arrived there" I think the future perfect tense is clearer. The second part in the whole sentence is just the "kommit dit" bit of this shorter sentence and the "kommer att ha" bit is implied by the first part.
The English version is a little similar. In "I will have called her before you have arrived there" there's a sort of implied "will" as in "you will have arrived there".
Not sure which bit you mean. "You will have arrived there" is perfectly standard English, as is Duo's English version of the full sentence. It would sound very odd if you put the second "will" into the full sentence, but I'm not sure if it's technically wrong. If you swap the order of the parts it's fine: "You will have arrived there after I have called her". (Although not a close enough translation for Duo to accept, I'm sure).
I was trying to explain that both parts are talking about the future, but some words are missed out because the second part depends on the first part. Sorry if that's not any clearer.