What's your mobile model no and network provider?
If you need to switch your sim card,you should unlock your mobile phone.You can unlock your AT&T locked mobile using network unlock code.It's the best way to unlock your mobile phone.If you need unlock code visit Unlocking4u.com Here you can get your unlock code to specify your mobile model no,country,network provider.After that they will send your unlock code with simple unlock instructions via e-mail.So you can easily unlock your mobile within a day.