I purchased a phone off Craigslist that claimed to be unlocked, but I find it isn't and the seller has dissapeared.

Anyhoo, my main thing is I want to be able to use my T-Mobile service here in the US with the iPhone.

I've checked the About area in the phone and these are the stats:

Version: 1.1.2 (3B58b)
Firmware: 04.02.13_G

I have no idea if the version is an upgrade or was it sold with it. Anyways, what can I do now to use my T-Mobile service... thanks for any help.