Go Phone SIM card in older Samsung phone, asks "SIM lock, enter password"?

I had an AT&T (Cingular) Go Phone, but the Motorola C139 I had broke. My friend gave me his old Samsung SGH-S307, which was on the Cingular network. When I put the Go Phone SIM card in the phone, a message immediately appears saying "SIM lock, enter password." When put in another C139 this message doesn't come up (so there's not really a lock on the SIM). I've tried 00000000, 12345678, the digits of the phone number, and most other default combinations. I tried unlocking the phone altogether, but the problem is unlocking the phone requires access to menus and a typing screen, which I cannot get to. They make go phones to work with only certain models, and my feeling is perhaps it just won't work with this old phone. But shouldn't unlocking fix this? Is there actually a passcode I need and can figure out? Like a PKU (I called customer service and there's very poor service to find out a PKU for go phones, it seems theres no way to talk to a real person to ask the question)? In short, is there a way for me to get past this screen to make the phone usable?