When I was catching, I called the majority of the pitches. Even if our pitching coach gave a signal, it was often left up to me whether to go with him or not. We had one particular pitcher who could throw what he wanted to throw when he wanted to. There were only two times our coach approached him about particular pitch selection. Both times were after he decided to not throw what the coach wanted. Both times the ball has still not landed..
The nice thing was that when I was catching, our pitching coach and I were usually on the same page as far as pitch selection.