Weird Southwest.com HTTP Issue

There seems to be about 10 years of this being reported, yet not very good answers.

Hopefully someone reading this has a good answer.

Southwest.com website from home and my office works fine and shows an IP of 208.94.153.100(Comcast Cable).

However from a clients office they can not access the site and the IP there is shown as 12.5.136.100 (Sprint T-1).

Searched the web the only thing someone reports as working does not work for many people and that was a netsh interface set tcp command.

Tracert, nslookup and ping all show the IP address as defined.

So aside from trying to convince Sprint they have a problem, with 1 website in the internet, any ideas how to get around this?

I tried hard coding the IP in the host file with no luck.
IPconfig /flushdns /register done, cleared cache of browser, tried all 3 browsers.

Any ideas or better Google searches than I are appreciated.