Mine has a stipulation about tethering too, but I don't care, I have turned off their ability to monitor in every way on my phone, besides the provider side looking into usage. My phone is setup to make the traffic appear to be going to the cell phone, the phone acts like a router. There is a file reference in the registry, along with some other keys too, and once they are changed, my unlimited data plan for the phone is technically all I am using. So I'm not worried. Hell, I am typing this on that phone as we speak, and my other computer is connected to the internet through a spare prepaid phone the same way. I have been running this way for over a year, and I frequently download movies using the connections. Some phones are better at masking than others. You might want to get a Mogul from Sprint, or a Verizon VX8300. The VX8300 is hackable using QPST, and the Mogul you use Resco Regedit. A simple Google search will provide you with the rest of the info you need. But you will need your MSL code, and I was able to get mine simply by asking. They didn't seem too concerned, hehe.
SIPPhone/Gizmo # 17476200648 / PIMPNET Chatline / Ran by Asterisk & Slackware 10.1.