at&t sim card in verizon blackberry tour 9630
 

I have an unlocked blackberry tour 9630 with verizon and I put my at&t sim card in it and it can only connect to the 2g 'edge' network, when i select 3g only it says "searching for umts". Do i need to flash/reinstall an at&t os for it to connect to 3g? when i put the sim card in my iphone it connects to the 3g network so i dont beleive its the card. any suggestions?