I did some quick research and it looks like the T-mobile SIM card will not work on an AT&T phone. There are several exceptions - hacks and having both phones and the SIM card unlocked. If I recall your BB is unlocked but you are using your original T-Mobile SIM Card.
The AT&T store should be able to transfer the data over (the T-Mobile store had a machine to copy the data from my AT&T SIM card over to my new T-Mobile SIM card).
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