Verbal communication is key once the ball is in the field. Here are a few that any umpire should know and use:
"I'm at third if he comes, Dan" and the "I'm at third, Dan" when he does come
"Catch, Dan" and "Down, Dan" on fly balls to the outfield
"I'm staying home, Dan" on those deep shots
"I'm on the line, Dan" when you can't rotate
"I'm going out, Dan" when you see trouble while in A
Like Thatsnotyou, I used to be skittish about talking to my partner, fearing it will confuse the players. As I grew, though, I realized that they had a completely different way of speaking to each other and they were too involved in doing what they needed to even process what Dan and I were saying.