When entering an office who should say "hi"? Should the person walking in "announce" their entrance or should the person already in the room greet the person coming in. For example I always thought it's up to the person entering the room or office to "announce" their entrance with a "hi" or "hello"

Personally, I am a 25 year old male who has just graduated with a Computer Science degree and currently a Software Engineer by profession. Recently, my dad was very interested in numerology after consulting a numerology expert, and he keeps chatting about it with his other friends over phone call...