Combination Internet and Telephone modems purchased or received in new or used condition from third-party retailers are not certified for use with the Cox network.
Panoramic gateways have to come from Cox. If you want to buy your own equipment either buy a separate modem and router(I suggest) or buy a non-Panoramic gateway. Basically it can't have phone service integrated into it. If you have phone service you can get a phone modem(eMTA) from Cox for free and then you can buy your own router.
The telephone modem would be a no-go because only Cox can issue telephone modems...not Amazon, Best Buy or eBay.
As for the Internet-side, subscribers have to rent the Panoramic and the previous renter of your device should have returned it to Cox. Since the subscriber did not return it, Cox probably banned it as either lost or stolen.
After following Mouth's advice, you could always use the Pano as an Access Point somewhere in your house.