how do I unlock a T-mobile phone to work with an AT&T sim card?

You can call the service provider T-mobile Network locked phone means you can call the T-Mobile provider to unlock it...But they will not give the code to unlock it.They ask to wait 30 days to get the code,Also ask more amount,Better get the unlock code for T-Mobile Network locked phones from here SimpleUnlocking and unlock it..Then use with any GSM Network like AT&T Network,also specify the Brand and Model of the Mobile Phone .