"Call 9-1-1" was never required on emergency vehicles, but it became common in the 70s and 80s as a means of educating the public about the 9-1-1 emergency number, especially as it was rolling out across the US. The public education need is long gone but it's now traditional to mark the vehicles with "Call 9-1-1". Thus, since many ghost graphics are just stealth versions of full color graphics, they include the "Call 9-1-1" part as well.
We're doing ghost/stealth graphics on all 4 of our new cars. We are also adding a Dial 911 graphic to the cars, but that is partially because being called a Marshal's Office some people get the idea that we're not actually sworn law enforcement, so the 911 graphics add some legitimacy.