The calls you get when the other end of the call is silent and after about 7 seconds they hang up. That's a telemarketers "mining" service. They use the numbers to verify its a valid number and they sell that to telemarketing companies. This weeds out disconnected numbers and fax machines.
I work for the telecom company that writes the software. You're welcome.
It's Rachel again, you know, from Card Member Services... Don't answer these, if you do, hang up asap. There's no poimt complaining to the gov because these folks are just criminals looking to get your credit card number so they can go on vacation on your dime! Now if the NSA was worth half the money we the taxpayers hand them they would figure out a way to stop Rachel and put her and her friends behind bars!