Back on 6 May 2017 CarolM (Cox Support Forums Moderator) mentioned on a different post (it's locked, so cannot reply there) http://forums.cox.com/forum_home/phone_forum/f/6/t/17648.aspx :

"If you currently have a different modem that you're using for your Internet service, you may use one or the other for your Internet service, otherwise to use both modem devices you'd have to pay for an additional line of Internet Service."

Does that mean:

If I continue to use my current modem for internet service (make no change to my current hookup), and install the new Cisco modem only for telephone (now I'm using both modems) I will have to pay for an additional  line of Internet Service?

or

If I continue to use my current modem for internet service (make no change to my current hookup), and hook up the new Cisco modem for both telephone AND Internet Service (use the internet jack on the router) I will have to pay for an additional  line of Internet Service?