I think this could actually mean "when will you answer?" Now this was described to me by a native german speaker, so take what you will from it. I was saying something like "Ich werde nach drinnen gehen" but meant that I had the intent of going inside, hence the "werde/will" (which in my english-thinking mind, also translates to expressing the "will" of going inside, and also that I "will" do it right now). However to a german speaker, this expression of intent is implied, and reinforcing it with a "werde" is unnecessary and actually misleading, pushing the event further into the future. My german phrase about going inside meant to her that I will do it at some point in the future, not immediately. I meant it immediately. She explained that when you're action takes place in the immediate future, you do not use "werde."
My point here with this is that "Wann antwortest du?" could mean two things: 1. When do you answer? -- which is a bit unnatural and I can only think of a few specific cases where someone would actually say this (eg, "at what point is one expected to answer in this process?"). OR the more natural translation 2. when WILL you answer? because in german the immediate future is implied in the context of the question.
I hope I didn't butcher that explanation. It's early. I'm only intra-coffee at the moment. Also I hope that's correct -- because she explained it to me in german and I'm #foreverlearning
Die Antwort (die Wort => die Ant+Wort) - the answer
Ich antwortE - I answer
Du antwortEST - you answer
Er/sie/es antwortET - he/she/it answers
Wir antwortEN - we answer
Ihr antwortET - you (plural, informal) answer
Sie/sie antwortEN - you (formal)/you (plural) answer
This is just a practice sentence. "When are you answering?" is how I'd put it in English.
I puzzled over this too. I wrote, "When do you reply." Which happily was accepted.
I wrote "When are you replying?". Not accepted. But the suggested answer was "When are you replying to?" It doesn't half irritate when a perfectly good English idiom is marked wrong and a nonsensical sentence given as a correct answer.
I struggled to come up with, "When will you answer?" and it was correct. But, the alternative answer was, "When do you answer?"
"When do you answer?" is a very unusually worded question. It's a very rare phrasing but I think it's a slightly more accurate translation than "when will you answer?"
"Someone must answer this phone." "When do you answer?" "When it rings, of course!" "When do you hangup?" "When they say, goodbye! Have you never used a phone before, Ted?"