putting sim card back into iphone?

[Sammiebby:]]

I had the iphone 3g for a long time and now recently got a new phone, and got the internet switched back to normal internet costs, not iphone internet. Well now i miss my iphone and want to start using it again, can i just put my simcard back into the iphone and start using it again? or will there be charges on the bill for getting back on the iphone?? Will at&t know i switched phones?