Assuming you're in the US:
1. Discussion of jailbreaking is forbidden here.
2. You can NOT downgrade the OS.
3. IF you were to find a way to unlock the phone, you will not get:
Visual Voice Mail
Support of any kind from any official/legitimate source.
If you are NOT in the US, contact your carrier to see if they offer unlocking. iPhones are supported on T-Mobile networks in many countries. The U.S. T-Mobile network is not fully compatible with the iPhone.