Me too (not that).
However, as a rule-of-thumb about devices: you can buy your own Internet modem; you can buy your own router; however...you can only get a telephone modem (eMTA) from Cox. Cox has to issue it. It must come from Cox. By law, Cox has to freely loan any equipment for telephone service.
To earn some rental cash, they'll combine the eMTA into an all-in-one device and tell you to either buy it or rent it.. However, they do have standalone eMTA. So, for example, if you've already purchased a DOCSIS 3.1 Internet modem and a WiFi router, Cox must freely loan you the eMTA.
If Cox doesn't have a standalone eMTA in stock, they have to freely loan you another device (2-in-1 or 3-in-1) to provide your telephone service.
Clear as mud?