Weird No Service Issue on 2G

[gold007eye]

So I went and did the jailbreak and unlock on a 2G iPhone yesterday that I received from someone who used to have AT&T and has that SIM card still in it. Before I Jailbroke/unlocked it it was showing AT&T with a signal on the screen and showing the emergency call screen.

Since Jailbreaking and unlocking the phone now I can't seem to get that same SIM card to pick up AT&T. It says "No Carrier" and when I look at the Network under the settings it is saying "Sim not provisioned". Am I doing something wrong or is this normal for it to display this after being unlocked if the SIM is no longer active on AT&T?

Also 1 other question.. now that I have unlocked it; should I be able to take a T-Mobile SIM out of an active phone/account and insert it into the iPhone and be able to use it to make calls, etc. immediately or does anything else need to be done first?

Thanks ahead of time. Done the iPodTouch jailbreaking, but this is my first iPhone.