In the G1 cartoon Peter Cullen only voiced Optimus Prime in the first 3 episodes. Due to a pay dispute he refused to do the role after that, but the soundalike they brought in still had to be credited as Peter Cullen due to union reasons and, ironically, Peter Cullen still got paid royalties.
The real Peter didn't voice Optimus Prime again until the 2007 movie, which is why he sounds a bit different in the movies than what we're used to hearing in the G1 cartoon.