Since people can cancel at any time, it's technically to your advantage to call back every hour. Every 5 minutes would be even better.
But considering what would have to happen for you to have an actual advantage, I don't know how much you improve your chanced calling multiple time a day.
The only time calling in the afternoon helps is if (1) a room frees up in the morning, (2) nobody is on the waitlist and (3) there is somebody else out there not on the waitlist who wants that room and is going to call that very day to book it. How likely is that?