"Wash the phone."
I guess it's better this way because you can't guess the meaning of the sentence. But it's so nonsensical, it's funny.
I should add that eventually even waterproof phones lose their waterproof properties. Mine had dust lodged in between the phone's frame and the power button, opening just the tiniest gap for water to sneak in. Every two years, it's probably wise to perform some preventative maintenance—taking the phone apart and cleaning the pieces with isopropyl alcohol. You'll probably need to replace the battery by then anyway.
Since this is the "Written" (해라체) lesson, why is this sentence in "Polite" (해요체)? Shouldn't "전화를 씻어라" be accepted?
It should be 전화기 not 전화, which I understand to mean "a phone call" not the actual phone device. Also, I don't think you would say "씻어" about an electronics device. You would say "닦아."
Wouldn't 전화기 땃그새요 or something make more sense. I mean, I might be wrong because I'm still a kid, but...