The effects of this are much larger than someone seeing your ad on a billboard and calling it right away.
It goes to name recognition, it shows success, it increases you brand value. I have people tell me all the time that they see my trucks all over town. I have had 3-5 trucks over the years with my name on them. Hardly a lot compared to the 80-100K people who live in my primary service area. But if your ad looks good and is eye catching people will remember it.
People will see the ad on the billboard and when they need something done and they start researching who to call they then see you on the net or in the yellowpages you will get the call. They will (maybe not even conciously) remember your ad or it will appear familiar and you get the call.
If you can get a billboard that cheap I would do it in a second.