blackjack samsung phone

Can a Samsung Blackjack 2 phone without SIM (Australia) be used as an AT & T pay-as-you-go phone?
I really want to buy Blackjack 2. The seller is willing to unblock free. I need to know if I encourage them to unlock, so you can use on AT & T pay-as-you-go plan. Do not I use top-up cards? That's what I want! If I do that, I still need a SIM card? Could someone explain please, help me. Thank you! : D
even if U is needed to unlock att … and just not say much about ATT slap charges on you Cuz … They did for me when I made my iPhone prepaid and you will still need SIM card with a GSM phone, especially AT & T i hope that helps
Video Unboxing of the Samsung SGH-i620 (BlackJack Slider)