I ran into a similar issue with Nextel a few years ago. (Not data vs. phone - but misrepresentation of service). My house is located right on the fringe area of most cell carriers. I asked them
specifically if I would have service out at the house and was assured that, yes, there was coverage out there. Aside from the other problems I had with Nextel (billing screw-ups, etc) I never got good service at the house.
These sales people do NOT know "beans-from-buckshot" when it comes to their service. All they know is they need to sell you a phone. It's like the healthcare system - YOU need to do your own research to make sure you don't get "stuck".
-CB