And a description of the car and (if possible) the driver.
Nothing's more frustrating than to have an actual assault or a hit and run or some other "real" complaint fall apart because the tag didn't match the car... or the return on the tag took so long that I sat in the intersection and watched the car go by without knowing it. We can often guess at common errors on tags (in VA, the letters M, N, and W are easily confused, for example) and find the right tag if it's close enough AND we know what type of car we're looking for. Even color and body style (sedan, pick-up, convertible, etc) may be enough.
And when you're not talking about a car... Address and physical descriptions are important. "I'm on Main near Cross" is a lot better than "somewhere on Main", and "101 Main, near Cross" is better still. Same thing with physical descriptions... "A kid" doesn't tell us much; "a white kid wearing a black jacket with a mohawk walking towards 101 Main" makes the person in question VERY identifiable!