Electromagnetic field and stuff. Also you might get ran over by a car if you read your messages while crossing the road ^_^
Yeah, it's a word. Dictionary.com lists "phone" as a verb as well as a noun, meaning that its present participle and gerund forms are also possible. It's not a good translation, however, because "phoning" indicates the act of using a phone, while the Korean only indicates that phones themselves are bad for you.
Since there is no 들, why can't another correct translation be "A phone is not good for health"?