I have apple iphone 3g and an old att sim card.bought the phone from ebay. i thought it would be just plug and play, but it doesnt. the seller told me that i need to unlock it, but what i know is that when you pop your ATT sim card to iphone 3g,it will work immediately. Does it mean the phone is not activated yet? If so, how can i activate it?thanks guys!
Btw, my ATT sim card is still working, i just wanna transfer the sim card from old phone to iphone 3g

Because the apple phone is from rogers so u need a rogers sim card account.
You will need to get the phone unlocked inorder to use your current sim card. you can get it unlocked at most phone stores/ kiosks in malls.