Chances are these scam realestate business do not have "clerks offices." The numbers on them are almost always local numbers and probably go to the personal cellphone of the jackass who hung the sign nine times out of ten. Legitimate realestate businesses usually don't need to scrap the bottom of the barrel like this.
The possibility of a false negative does exist, but the possibility of a false positive seems greatly overstated and I do not believe aversion to false positives motivates the lack of enforcement as was suggested above.
And because it's highly improbable that "yes, those are my signs" over the phone is enough to result in an infraction if they did.
Chances are, you're not even going to get the phone call in the hypothetical you're propping up, even from a clerk's office. If your name and phone number is on it, you'll likely just end up getting it in the mail without even the courtesy of a phone call to ask how your morning is going.