Unlocked iPhone 3GS 4.3.5 Firmware - Invalid SIM/SIM Failure

[canon10d]

Unlocked iPhone 3GS 4.3.5 Firmware - Invalid SIM/SIM Failure
Just as the title says, when I insert my T-Mobile SIM card I get "Invalid SIM" and sometimes "SIM Failure"... It shows 1 bar of service with no text next to it and when I try to make calls or send messages they fail.


Any idea? Thanks in advance