Since you have mentioned that your curve is unlocked, did you try using someone else's SIM card (preferably of a different provider) in your curve?
Also I assume that voice mail is deactivated with your number since you got the message "We are sorry. No one answered the call. Please try again later". Had VM been active, it would have gone to your voice messaging system. Here's what I would have done if I were you..
Step 1 - In order to wipe the doubt that nothing is wrong with the SIM card I would have got myself a duplicate SIM card from a T-Mobile store in town. I would observe the behavioral pattern of the new SIM in the curve for 1 or 2 days. OR if you can manage to get another cell phone try using your original SIM card in it.
Step 2 - Setup your voice mail and activate appropriate settings in your berry to redirect calls to your VM if you either miss a call or disconnect the call. The moot point is if you do not pick the call, the calling person should be redirected to your VM. Get that checked from T-Mobile rep.
Step 3 - Revert back to the previous version of OS which was loaded on the berry. I have seen people complaining about OS 126.96.36.199 from released by T-Mobile but that is sporadic. I have myself used East Asia version of OS 188.8.131.52 (not T-Mobile) with no such issues.
Try these and let us know how it goes...