State Bird Provisions however has so much demand that there is a line for walk-ins hoping for a cancellation an hour before the restaurant opens and they appear to not actually hold back much (any?) inventory specifically for walk-ins.
But generally speaking yes, you will often be able to call a restaurant and get a table or possibly walk in even when their online reservation system shows up as full.
Also, you can also put yourself on a waitlist at many restaurants. If someone calls to cancel, before they release the reservation back to the online system, they'll go through their waitlist and call and ask if you want it instead.