I know quite a lot of people who use no contract sim cards, because they think that thanks to this, nobody will be able to learn their name, address or any other personal information, just by using their phone number. They believe that if there's no personal information associated with the number in the carrier's database, nobody will be able to find it using for example a cell phone lookup. Unfortunately, they're not right, buying a prepaid cell phone will not make you any more anonymous if you decide to use the phone number just like any other. This means that you will be for example shopping online using your personal information and the prepaid cell phone number or you will take part in some online sweep-stake where you have to provide your cell phone number. If you use the number in this way, you can be sure that your data will be collected and aggregated into a cell phone directory with all your personal information in it.
However, there is a easy way to prevent such situation - you just have to read the terms of service before you enter your personal details. If you notice that you agree to resell your data, you shouldn't agree, since you can be sure that after some time the information you entered will be bought by some cell phone directory and everybody who pay a few bucks will be able to find your name and address just knowing your cell phone number which was meant to be totally anonymous.