HI if the iphone is locked to some other network provider then sure it wont accept your t-mobile sim until you unlock the iphone ...You can easily unlock the iphone using sites like OnlineGSMUnlock.com here they are rendering remote unlocking service to unlock all model iphone from network lock ...You first check that whether the service is available for your country and network provider .If yes then pay for it and unlock your iphone then freely use it with any GSM sim .......