Jettie...
I'm having the same problem that you are describing. I had no issue with my iPhone 3G, but my iPhone 4 will say it has a full 5 bars of service from my M-Cell and consistently display "Call Failed" when I try to dial out. When someone tries to call me, it will say "Subscriber is unavailable", or something like that. Any clue on this at all?
I have the exact same problem 😡