If you unlock the T-Mobile Blackberry, You can not only use AT&T SIM but you can use any international GSM SIM to it. You can unlock your mobile using unlock code. You can get the unlocking code from Theunlockarena.com at affordable cost.
You can also search for your cellphones unlock code and go to mobileunlock.co.uk. It will ask you for your phones IMEI number and walk you through the steps. But I think the charge like $15.
You need to get it unlocked at your local phone store it costs about $25
but it has been used before