vB Easy Archive - unlocked ATT smt 5600 with Cingular sim can no longer stay on network

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unlocked ATT smt 5600 with Cingular sim can no longer stay on network
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Posted by: johnnyboy0527

Hoping I'm not the only one with the problem as it's been a real pain to try and trouble shoot.
I purchased an unlocked, ATT branded smt 5600 for my wife and set it up with her Cingular sim. It worked beautifully for a couple months but 1 day (about 3 weeks back) the phone lost all bars and kept switching between "no service" and "Denied, Searching...".

Here's what I've tried so far:
hard reset -*no effect*
swapping in my Cingular sim- (I have the "older" version while hers is the new 64k)*same behavior as hers*
I put in a TMobile sim- *it worked perfectly! 4 bars and never dropped*
I acquired another Cingular sim and tried it - *same behavior as our sims*
I switched the network settings to "manual" and specified "ATT"- *got full bars and access for about 60 seconds, then it dropped off*
I switched the network settings to "manual" and specified "Cingular"- *same as above; 60 seconds of success followed by failure*

Outside of finding an ATT sim to test with, I don't know where to go next. Does anyone have an idea of why we had but then lost network access? And, more importantly, does anyone know how to fix it? Thanks in advance for any assistance.