If someones ringing you its either to chat or to meet up now or later.
A meet up call takes two minutes so whats wrong with doing it in company its better ignoring another person whos trying to talk to you. If there calling to chat just say I can't talk now.
Leaving a message costs money for both parties. And the message will just be "Call me. Its complicated". The person should just call you back.
My phone manners are really the opposite of this. I think its rude and annpoying when people don't pick up and don't call back. Its like if you tap someone on the shoulder and they ignore you.