I would leave a VM message the same day (even middle of the night) so they know you're interested. Then I would keep trying daily or several times a day until you get hold of someone. If you absolutely can't get hold of them, send an email.
It would be a stretch to say you couldn't find time to call them back over several weekdays. Take a few minutes (and a delay if needed) and go in a bathroom stall. The only plausible excuse for not calling within one work day would be military deployment to somewhere austere.