Just a thought, maybe your phone is roaming on an unfriendly network and T-mobile is restricting your access.
I have T-Mobile service. My phone was a Sony Ericsson T300. It worked fine until recently, now it is acting very weird.
I go into areas where I can have even all 5 bars, but instead of the screen saying "T-Mobile" it says "No access". Does anyone know what this means? Sometimes it then jumps back to T-Mobile. I called T-Mobile customer service and even they didn't have an explanation for the "No Access" message with the bars showing. I think this is why I'm getting far more "dropped calls" than I used to.
Anyways, I have three other phenomena that occur:
1) I have several bars of signal strength, but calls do not ring my phone. At other times calls do ring my phone with the same or lower signal strength.
2) I can be in the middle of a call and suddenly I can still hear the caller but he/she can no longer hear me.
3) My screen says "T-Mobile" and shows 4 or 5 bars of signal strength yet I cannot make outgoing calls. This situation is random, about 2/3 of the time I CAN make calls.
T-Mobile says it is not the service and I honestly do think it is the phone because I moved my SIM chip into a different phone (a Samsung R225M) and no longer have any of these problems.
But I want to understand why my phone would suddenly do this and is there anything I can do about it, or is my phone essentially "dead" now? Please let me know, thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic