Is it legal to share a phone with roommate?
you do pay the monthly charge on time, who care how do you use it?
I'm using Pacbell (SBC) DSL, Charter Cable (by Earthlink), with router, both don't need to clone the MAC address.
You get 1 IP if you choose the basic plan, the router gets 1 IP - that exact IP (either dynamic or static, doesn't matter), they have no right to forbid you using router. The only drawback is you can't get tech support from them on your "router" unless you buy from them at a rip-off price. If you know how to use a router, I don't think you need tech support from them. Their tech support is a joke, they don't know anything about computer, they only can tell you to try restart the computer, reset the modem, etc, and file a technician visit order if they can't solve it by phone. Once a technician came to my place and saw my "router", he blamed it's my "switch" causing the problem.
Once I moved home, the old home was using Pacbell DSL by a separate line. The new home's old owner is using self-install dsl so using the same phone line. Pacbell wanted to charge me $150 if I want them to install the DSL into separate line or sell me a self-install DSL kit for $200. You can buy such self-install DSL kit for $30 shipped from internet. I just borrowed some extra filters from friends who're using self-install DSL.