Insert other GSM Network Sim card in your i-phone..It's locked means it will not accept other GSM Network Sim card..It's Unlocked means it accept any GSM Network Sim card...You can Unlock GSM Network locked I-Phone..You need to specify the network and country it locked now..You can get the Remote I-Phone 3G unlock service from here Mobile-unlocker.com/apple-iphone-3g/rs2wp9/ and Unlock it...