When you call someone, it is polite to identify yourself. For example, if your name is John Smith and you were calling Robert Chen, you would say "Hello, this is John Smith. May I speak to Robert Chen, please?". When you answer the phone, it is ok to answer just "Hello". After your caller introduces himself, you would say one of the following:
"Hi John, this is Robert. How are you?"
"Speaking." or "Robert Chen speaking."
"I'm sorry, but Robert is not able to come to the phone right now. May I take a message?
"It is not polite to call someone before 9 am or after 10 pm, unless it is an emergency.