Please help! AT&T switching SIM cards?

My dad recently got a new phone on Verizon but the rest of my family and I are still on AT&T. I recently broke my phone so when my dad got the new plan he gave me his old blackberry. I put my sim card in (I am sure it is not broken-have been using it in a different phone) and the phone does not seem to register that it is in there-none of my contacts are showing up, and there is no service (where the bars usually show up it says off). What do I do to fix this?