Find IP address away from home.

Do you have any remote access to on site? If not, and I assume your not using the Cox Home Network Manager, then only way to find out is to call, or some other way of contacting a Cox employee.

I mean if you were in a pinch, and new what one of your last IP's was, and your router was set to respond to ICMP or some other port, you could do a port scan of IP block, but that would take hours and probably give you false positives. So yea..probably have to call. 

You could resolve an IP address with a MAC address; however, this privilege is limited to Cox technicians through the Address Resolution Protocol (ARP).  You would need to get the IP address after booting your home computer before leaving...or call whoever is at home to get the address after booting.

You could install Network Management Software on your computers.  This Simple Network Management Protocol (SNMP) includes a network discovery feature to request the IP and MAC addresses of its agents (devices).  Perhaps there's a reverse option for the agent(s) to automatically send their address information.  However, if the IP address of your travel computer keeps dynamically changing, this wouldn't work, unless the agents could send their discovery information via e-mail.

I haven't use Network Management Software in years, so I can't recommend a vendor or if these options are available.  You'll have to research.

Well I think the whole point was to do it without onsite access. If he could call the location to get the MAC then he could just ask them to go to whatismyip.com and that would tell him his IP. Not to say you don't have good ideas,  but i think they are more long term then short term. With that said, you can find the Mac of the CPE under MCA in internet tools. But like you said, to use that you would need to call Cox, and if you did that, you could just as easily ask them to look at your modem and just see what IP it's giving out.

I think the whole point is the IP is hard for someone to find out unless they have access to the account or on site. I think this is very much on purpose, otherwise you could figure out someone else's IP and do a DDOS or port scan (to see if remote access is enabled) or other shenanigans. Not saying that is what is happening of course, just saying it could. At this point, its simply intellectual debate. I think the OP has been answered.