I wish I had the link for you now, but I read an article at one point by a guy who made those websites that get you to opt-in to a horoscope subscrption to your phone.

His finidings through testing were that when he requested a zip code up front, his initial click-through rates were lower, but his response rates on the back end were much higher. It turned out that people who gave a little information up front were much more likely to give a lot more later on. It really boils down to trust.