If you're using a VPN do disguise your Internet connection it's possible a security feature of the website is being triggered thinking that somebody is using a stolen password to try and access your account. You can try signing into your http://www.cox.com billing account and see if you're prompted for secondary authentication. If so, you should be able to complete the steps to temporarily authorize the VPN connection to access your account.
It's possible that the IP block that is assigned to your VPN is blocked by a SPAM list. By checking the IP addresses against the site that Tiffany previously mentioned, we will be able to better determine what is causing the issue. On a side note, are you able to login to your Cox account on a different device while using the same VPN connection?