Not stupid at all. They ask because a very small percentage of recipients are stupid or naive enough to actually give out that information. That's why they keep calling.
Called my cell, answered. Unidentified company - ROBOCALL offering a free, all expenses paid cruise if I press "1" to take a short survey. Press "9" to be "permanently removed from our call list". my number has been on the FTC Do-Not-Call list for over 5 years! Filed a complaint on the FTC web site.