Is it improper etiquette to say "what's up" on the phone? Is this phrase different to different age groups?

When you're talking to friends, you can answer the phone in any way you like. When you're talking to an older person, or if you don't recognize the number, it's better to just say "Hello?" to avoid confusion. And if you're the one placing the call, saying "What's up?" right off the bat doesn't make sense, it seems like you're jumping right into the conversation without going through the preliminaries, such as greeting the person, etc, before you get into the reason for the call.