This article address the differences between the two carriers and how to access features like call forwarding, Conference Calls, and many others. Good read.
ttp://www.appleinsider.com/articles/11/02/16/apple_details_gsm_cdma_iphone_differences_att_ceo_disparages_app_store.html
ttp://www.appleinsider.com/articles/11/02/16/apple_details_gsm_cdma_iphone_differences_att_ceo_disparages_app_store.html