I'm pretty sure this is due to your device running a custom firmware by your phone carrier. Therefore, you're gonna have to wait until your carrier releases a new version or you can try to manually update it (called flashing).
HOWEVER, I am in the same situation as you are and I tried to flash my phone to 2.3.5 but my SIM card seemed to require the custom firmware to be working. Unless you are able to find the exact firmware from your phone carrier, I wouldn't recommend flashing, you might end up in the situation I was and I had to bring my phone back to the store because I couldn't get my hands on the custom firmware.
