answering the telephone, Ana told me to keep it quiet. The ”it” does not need to be there. One can simply ”keep quiet”.
This sentence is incorrect in English, unless it is Ana who is answerimg the telephone. I believe Duolingo needs a serious check by linguists.
What makes you believe it's not Ana answering the phone?
Does "...Ana mi-a spus să mă liniștesc" mean: "Ana told me to be quiet"?