If I am correct (and there are times I am not!!!) yes, you need to call at 30 days. Once 7 days hits, there is no waitlist anymore and you need to call daily to see if you can get something.
Someone will correct me if I'm wrong, but I might be right on this one.
If your waitlist was setup to end at 30 days out, you would need to call back before then to have it changed to 7 days out. You can have the waitlist setup from the beginning to go to 7 days out. I always have them set mine to 7 if I waitlist.