Cox called me offering a FREE modem upgrade, at no cost to myself and then charged me for it

I got a call about a month ago, with a guy telling me cox wanted to send me a modem. I asked what it would cost, or what the conditions were, and he said it was literally free and cox was sending them out to customers who wanted them.  It literally sounded like some sort of scam, to the point where I google it afterwards and other people had gotten the same type of call. Now this month I see a 180$ bill on my statement?!  I haven't even opened the thing, and the modem I have right now works perfectly fine. What I am really angry about though is that cox would try to scam me into buying one of their modems by literally telling me it was free, then just tacking the cost of it onto my bill. This is ridiculous.