I don't think I realized this, but the US Do Not Call Registry has a built in expiration of five years. According to this Newsweek article, the reasoning behind that is because people who move or change numbers.

For what it's worth, the US Do Not Call Registry is now about five years old, so numbers that were entered in when it first started are now (or soon) expiring. You can use the site to validate your phone number, or to enter the number in the database. I re-entered all three numbers (home, Jen's cell, and my cell) just now, even though I already had each number in before at different times. Now they all expire on the same day - September 5, 2012.

(And yes, because I'm practicing my Getting Things Done techniques, I just made an appointment for myself in August 2012 to renew this. /me is a geek.)

Although, technically it's a task and I shouldn't have made an appointment...