I would recommend a reset as well. What you could do before the reset. Give a call from ATT, and make sure the IMEI of the phone is registered properly and let the rest look over the configuration of your account. I wonder if the store representative rushed a bit in its configuration.
Code:
Menu> Settings> Privacy> Factory data reset.
No need to do the same with the memory card if you have one just the phone You may want to back up your list of applications with an application called Dragon application and backup SMS and possibly some call logs before restarting. There are some useful applications for this in Android Market.
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