Hey everyone, my carrier is Straight Talk using SIM. I was using my Samsung Galaxy S3 up until the phone started to fail. I just transfer over my sim card over to my S4. Both are T-Mobile phones. For some reason when I try to make calls I get an error message stating "Not register on network". I even tried to unlock and root my phone, but I am not savvy to attempt anything further without help. I am not sure what the problem is, but the phone is in new condition. It is either the service or the OS of the phone. Can anyone help me?