Most phone calls on TV are Important Business: to relay important news to a character. On doctor shows, this usually means the patient is coding. On cop shows, there's been another murder, so the guy the detectives took into custody must be innocent. In any event, the character must abruptly end the call and rush off to Deal With the Matter. No time for such pleasantries as "goodbye."
The phone will always be answered with "Yeah!" or "What?" or "Character's last name." Never "Hello."
In real life, though, most of us still end phone conversations with "goodbye." However, with caller ID and customizable ringtones, "Hello" isn't the universal phone greeting anymore. It could be "Hi, Mom" or "Hey, whassup?" or "Mornin', sweetie," depending on who's calling.