Forum:Qui-Gon's Force Voice

I know there are Force Ghosts. But Qui-gon only returned as a voice. In Attack of the Clones he says "No Anakin! No!" and in Revenge of the Sith (Deleted Scene) He talks to Yoda as a voice. So the first question is: How did he return as a voice and not a ghost. And question # 2 is: Sense Obi-wan & Yoda learned to become a force ghost, did you need to learn how to become a voice?