At&t sim card in a unlocked phone

I bought a unlocked blackberry off of ebay put my at&t sim card in it. the phone showed that my sim card is in and it showed me my mobile number. The problem is it will not allow me to make any calls or do anything it says no service even though I have a At&t plan and everything it still does not work. Does anyone know what to do in my case?

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You might have purchased a GSM Blackberry without the frequencies used in your area or the network setting internally might be on manual for band selection and/or network. Check the settings and put both the bands and network on automatic if that is the problem.

thats the model number of it?
are both 850/1900 bands on it?

Yeah check the bands, I recently bought an unlocked phone and tossed a new 3G sim card in it and it works fine.