Security Deposit Check Expired

I have two checks sent by AT&T for returning our security deposit when we first opened two new lines (when our company was relatively new). I forgot to make deposit of the two checks, and they already expired... I tried to contact the customer service and the billing department many times, but always got disconnected... Any suggestions to resolve this? If there is no other solution, my next step is to get in touch with the dispute resolution for this issue...