Erm... This device?
Nokia N800 - Full tablet specifications
It doesn't take a SIM. It has no cellular connectivity. It runs a tablet version of Maemo.
In terms of SIM, there is not ATT & other. There are SIM sizes: mini/micro/nano and any carrier might be likely to run any size, be they ATT or T-Mobile or Vodaphone or whomever.
Did you mean you got a Nokia
Lumia 800?
Nokia Lumia 800 - Full phone specifications
This device runs Windows Phone 7.5 (upgradeable to 7.8). It takes a micro-SIM which is the middle size. The radio is suited for AT&T or any AT&T airtime reseller.
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