You need to Unlock I-phone from network lock to use other GSM network Sim card....You can get the Remote unlock service at here Wickedunlock.com/apple-iphone/rs11wp9/ and unlock it from network lock.Then use any GSM Network Sim card in the Phone........If the Phone is already unlocked means you can use with T-Mobile..It's locked means you need to unlock it and use with any GSM Sim card like T-Mobile.