Missed call % factoring in calls even when on DND, would like that to be taken out of consideration on Performance Reports

My company is really trying to dig into and monitor track our employees missed call percentage. So i have been pulling performance reports that have the "% Missed (w/VM)" stat HOWEVER we are noticing this stat is even docking the employee when their phone is on DND (do not disturb). If an employee is on DND say having a day off or on an important conference call or in a training, then we do not want to have those calls count against them. We are only trying to track when they truly were available but missed the call because they weren't quick enough or maybe because they just didn't feel like picking up the phone. That is what we are trying to measure and report on.