It is required to have a mac to build iphone application or we can use VirtualBox to emulate the MAC OS X platform?
It is required that you be running OS X and you can't virtualize OS X on non-Apple hardware (by terms in the EULA, not for any technical reasons). Therefore it follows that you need a Mac computer to build iPhone software.

See this question and answer for a discussion on virtualizing OS X: Is there a way to evaluate OS X without purchasing Macintosh hardware?