My problem is almost the same I never realized it because I am almost always on WI-FI but my wife noticed it right away and I have not heard the end of it. Using 3G is a nightmare everything times out and she cannot make calls.
I noticed that even the phone shows roaming on call and data, it still uses Sprint 3G Data and minutes. Sprint 3G Data Roaming shows 0. It might be some kind of software problem.