Bought a phone on eBay and its activated?

i bought a iphone 4 for $400.00 with a "bad esn" and when i got it...(Month ago) i noticed i could place calls and text. Well its been a little more then a month and it still has service. What do i do? Am i going to get charged for the time i've used it? Its fully activated. If its considered lost or stolen and cops are involved am i going to get charged? i bought this phone like this on eBay.