At DFW a couple of years ago, the policy appeared to be that they'd put you on a standby list if you walked in; and then you'd get in only if there were a no-show.
We just showed up. They threatened us with never getting in, but then after a little over an hour there was a no-show and we got in. I didn't call in advance because I knew for sure what the official phone answer would be.
These days they could be stricter.