Ooooooooooo I like that one.
Another think you can do is answer the phone and after he intros himself, simply tell him you'll be right back and go about your business.
Make it his decision to not follow up with you.
That one is posted in several places up here in the mountains of NorCal. This is a very rural place, and guess what? People here MEAN IT. From My first day up here it was explained straight off: "You can travel along any road, paved or unpaved, that is not blocked off in some manner. The highway is of course open to all, and the other roads leading off it on either side are generally Forest Service roads. Some are Logging roads, but they will have a crude No Trespassing sign of some sort posted right at the entrance. They do not lead to any areas of interest -assuming you are not part of the logging crew- and the logging truckers will be mightily upset with you if they are bringing a load out and you are in the way. If you decide to go exploring along the roads out there (outside of town) let someone know before you go so if you do not reappear after a couple of days we will know that you probably went down one of the roads with the "NO TRESSPASSING. VIOLATERS WILL BE SHOT" sign on a tree and a smaller "SURVIVERS WILL BE SHOT AGAIN" sign below it. People MEAN it. We will probably find your truck eventually, but probably not your body. There are just too many places out there to get rid of one." (This area is in the middle of some huge National Forest lands).