I wrote "no one called," and was corrected. Instead they suggest "no one was called." What indicates passive voice here?
Nothing. Your solution is perfectly fine, it should be accepted. "Telefonálni" can mean both "to make a phone call" and "to have a phone conversation". My first thought, reading the Hungarian sentence, was "No one called.", just like yours.
No-one phoned was not accepted. Reported.
Ezt inkább így mondjuk: "Senki SEM telefonált."
Senki se telefonált. Nem telefonált senki. A sem a nem is összevonása. Sem is made out of nem + is.
neither was accepted: 'nobody called on the phone' . No one called is way more general, like in: "I stayed all day at home; No one called."
"Nobody was talking on the telephone." Not accepted.