It depends on both the horse and the time. Some horses, as said earlier, call once and keep working. I don't mind with these types of horses. Other horses (like my mare) complete lose their focus. I don't allow my mare or others with like personalities to call at all during "my time".
My horses have two time zones: "my time" and "their time". During their time, I don't care what they do. They can buck and prance and pace and call. Their time occures when they are in their paddocks or stalls, alone, without a human paying them any attention.
My time is any time they have on a halter, tack, or I am doing anything to them. I can be petting them, riding them, or leading them. When I'm around and handling them, the horse isn't allowed to call. I want the horse focused on me, not his friends.