just bought used BB 8100 E. the phone says cingular/att. but my SIM is not recognized.
i looked in the settings and found that the network is on T-Mobile!
how is this possible? and is it the problem my SIM is being ignored?

can someone walk me through a fix, re-flash or whatever?

So the branding says att, but its locked to the t-mobile network?
You need to get it unlocked.

Contact Horizon Wireless Online, one of the Forum sponsors for an unlock code.

FYI, fastest way to handle this situation is go to a local T-Mobile kiosk/Brick&Mortar location. They are only ones who may handle this situation quickly and simply.

THIS APPLIES IF AND ONLY IF YOU ARE A T-MOBILE CUSTOMER for >3 MONTHS!!!!!!

Wuzzer... as your signature suggests... I'm going to "take it up with you". You're certainly not pissing me off, but rather you're setting yourself up for a flaming, way too often.

You're new here, and by your own admission you're still learning.
I suggest that you complete the learning part before you start the posting part.
A quick review of your recent posts makes it plainly obvious that the learning isn't quite finished yet.

Sorry WUZZER, but you need to do some more research. There are many who can unlock the device.

In some cases, the carrier will. In others they may not.

With this device, since it is an ATT branded device, going to TMO will not work.

Seems very strange that the phone says ATT, the internals say TMO and your ATT Sim doesn't work.

I assume that you bought it used?

Might have has the housing replaced. Your SIM could be bad or inserted incorrectly.

Have you tried a new SIM and inserting it differently?