You don't mention what kind of cell phone it is, but I'm guessing an Apple iPhone. Apple added a feature called Activation Lock to combat the rampant theft of iPhones. This prevents stolen iPhones from being used by requiring that the legitimate owner remove the device from his or her iCloud account. If they do not do this, the iPhone cannot be activated again.
Sometimes owners don't do this and it can cause problems. More often, the reason an owner can't be contacted is because it is a stolen device. It was a great move by Apple to remove the incentive for people to buy stolen iPhones.