Network Locked Sim Card Inserted

I am trying to switch to mint mobile from At&t, but the phone is telling me that the new sim card from mint mobile is locked and that I need some sort of network code to unlock it. I asked Mint what to do and they said that I have to contact either At&t or the manufacturer to get my phone unlocked. I cannot call at&t because my phone number has been switched to Mint and I cannot place a call without the sim card. I need to get ahold of someone to get my phone to work.