Keep a CLOSE eye on him, watching for signs of intestinal blockage. Signs could be lethargy, not acting himself, not eating, and unable to pass stool. It could help that it was a old brittle rubber band, but rubber bands traditionally are dangerous for cats. If you see any of the above signs, or anything that doesnt seem right, get him to a vet immediately. Regarding calling 911, thats funny! Unless you are in a very small community where everyone knows each other, and is willing to help one another out, 911 will refuse to respond on a house pet. If it didnt block her airway right when she ate it, choking isnt going to likely be a problem (unless she tries to vomit it up and it gets stuck). The most likely problem will be the intestinal blockage.