"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.