what the OP is referring to doing is what I called the "best buy mobile" scam. There was a certain point in time, and maybe people still do it, where people go to a place like best buy or walmart, start a new line of service, then call Verizon/att/whichever carrier, and claim to have cancelled service and returned the equipment. The actual carrier doesn't know whether or not you have actually returned anything, since the big box stores use a different computer system. So then the carrier cancels the contract in the required time frame.
To my knowledge...best buy and walmart are both on to these things now, along with the carriers. So I would definitely NOT do this.