I think it depends on the type of your work... and also on who is calling. When your boss is calling - better answer the call. If someone from your close family - it may possibly be the emergency situation. When you don't answer you start thinking and getting nervous. So your further work becomes ineffective.
When you just talking to a client / customer or when your work demands constant attention and every distraction may cost people's lives - you'd better turn your phone off before starting your work.
one of the professors was talking about some more restrictions in America for not answering any calls or messages in their working hours which isn't the same in Iran with such restriction...so I just wanted to know whether is it true or not and how it goes in other countries