(potentially) stupid question

[Duke of prunes]

So I have a jailbroken iPhone 3Gs, 3.1.2 bought off eBay, which has a T-Mobile lock on it. Thought if I just got myself a regular T-Mobile sim card(just arrived), I'd be able to use my iPhone to make phone calls, but it's giving me "no service". I used Pwnage tool to jailbreak, and as far as I know I didn't upgrade the baseband. As you can probably tell I'm quite new to all of this so can anyone help me out?

Thanks

[nachobel]

Go to Settings -> General -> About, scroll down all the way to the bottom. What number is next to the "Modem Firmware" setting?

Second note: Jailbreaking does not unlock your phone.