I wonder how many calls they have left go go out. It can't take that long to call 500 or so acceptees if you have a decent number of callers. If each call is roughly 5 minutes long, then 1 adcom can make 12 calls in an hour. If they have 10 adcoms calling (but this is the number that I really don't know), then we're at a roughly 4-hour-long window. If they take a lunch break, then perhaps they will finish around 2-3. There are a lot of assumptions in here, though.