GTA 5 From alternate reality to game character: Trevor Philips Real Voice vs Steve Ogg

Trevor Phillips came from the alternate reality who got transferred into the game with his real voice because

I know because my true identity being just like his and then what else Steven Ogg only voice him during the missions in story mode except in something sensible and free roam

Trevor Philips came from the alternate reality and got transferred into the game with his real voice. While Steven Ogg only voices him during the missions in story mode, except by the book