How would this even work? Say I decided to buy two billboards with different designs selling the same product, and put a different phone number at the bottom o

How would this even work? Say I decided to buy two billboards with different designs selling the same product, and put a different phone number at the bottom of each. Looking at my phone bill at the end of the month, I have a count of how many responses I received from each billboard. Have I just committed a crime?