I just got the new sim and now everything is working fine!
Great news! Thanks!
3.1.2 Firmware, iPhone 2G, T-Mobile.
iPhone 4 on the way.
Add some rep if I helped you please!
Any T-Mobile Store will have the updated sim. You can also go online and buy it from a 3rd party vendor. If the sim is relatively new (approx. less than 2 years old) it should be fine.