Translation:Wash the phone.
Did this question get moved from a category or something? Nobody is talking about the polite sentence being in the "written" lesson?
The suffix ~기 is often used to denote machines, but 전화기 is actually a "telephone set", so the receiver, mouthpiece, and base together. 전화 by itself can mean both "(tele-)phone" and "phone call".
The 를 is also there to denote it is the object of the sentence and clarify you're talking about a physical phone, not the act of making a phone call.
Im curioua about this as well as i was taught 전화기. I know 기 can be used to denote a machine but i dont know if its optional
Koreans do not say this expression. Duo Ringo's expression is bizarre. '전화기를 닦으세요.'
But would they write it? Apparently this lesson is specifically about written Korean so I guess some things must be different...
"...the other two-thirds of the population stayed on the planet and lived full, rich and happy lives until they were all wiped out by a virulent disease contracted from a dirty telephone."
I'm pretty sure "전화" only stands for "call," you have to add "기" to make it mean "phone." Only saying this so Duolingo's translating system gets enhanced. Again, I'm not 100% sure; just a thought. Edit: I've seen some other mistakes too, some relating to grammar and some just not making sense. I already a lot of Korean, thus am able to tell these types of stuff; I'm only here to try to get better at it.