When to harrass worker (nicely of course)

Our worker is HORRIBLE about responding to emails. I often send an email and wait and wait and wait..But then I call and leave a VM and she calls back the same day. CRAZY, but I can only imagine the number of emails they get, and sometimes it is easy to let one slip through the cracks or procrastinate on replying due to other more urgent matters. I would just call and leave a nice message asking the status.