From your description, sounds like your phone is unlocked. You might only need a new Cingular SIM after your account is migrated.
I'm in the process of the Cingular takeover of my service provider (SunCom)and want to hold on to my phone since the ones offered are somewhat of a downgrade from what I have. As a result, I figured I would check it by inserting the SIM card from a friend's phone who had already made the switch. When I put it in, it read cingular on my screen, but also had the little roaming pyramid shown. I even made a call with that card. I think it acts like it's unlocked, but I don't know why the little pyramid is there. Can anyone say for sure whether they think it sounds as if it's unlocked?
BTW, I put my SunCom SIM in my friend's phone and got a screen that said "Enter Subsidy Password", which tells me his is locked,I think.

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