Nokia Lumia 800 - Why my Nokia Lumia gets crackle sound with headset ON?

This behavior is identified, and it occurs if TTY/TDD mode is changed to Full in Ease of access menu. Go to Start > Settings > Ease of access, then select TTY/TDD mode OFF.

If you want more information about TTY/TDD mode, refer to the Windows Phone support site, How to use my phone with a TTY.