Getting out of his seat, calling out and acting silly are not things that should warrant a call home in the middle of the day for a second-grader. I agree with OP on that. What the teacher should have done if she was seeing this too often is to email you or ask for an in-person meeting. If this happens so often that is disruptive, the next step for her would be to call an emergency intervention meeting between you and the school where the counselor, administrators and others would get together to create a plan.
BUT a child walking out to the field at school when he's not supposed to is elopement and it's very serious. Schools are not equipped to handle that. To bring him back the teacher or someone else would have to disengage from whatever they were doing to chase him down and bring him back. This puts other children in danger and should not be happening even once. If it's happened more than a few times now you need to start taking action to stop it. I know of a few cases where children had to be moved into special programs because of elopement issues. You need to not be at your computer complaining and figure out a way to get on top of this.
Pretty much agree that a call in the middle of the day might not be warranted-unless it is the teacher's break. However, the teacher may want to talk to the parent when the child is not present. Then, when would you have the teacher call? First thing in the a.m.? Late at night?
It sounds to me like the teacher is frustrated because the child is disobedient. "Acting silly" is a call for attention. If he constantly disrupts the class, that is a problem and can be quite frustrating--it takes time from instruction and may disturb the whole class. And, as the PP said, the "elopement" issue is quite serious.
This sounds to me like more than ADHD.