IPHONE 4 WHITE 16GB !

White apple iphone 4 16gb still has protective screen on it so not a mark on the screen. Few marks around corners of the phone nothing to noticable in a good condition. Phone is on t-mobile but when i put a t mobile sim card in it comes up with signal but will not make calls unlike when a phone is stolen and it says no service so i am confused as to what is wrong with the phone. Anyone with a bit of knowledge should be able to fix it. All the phone works perfectly. Text me for any information as i will probably not be able to answer The Phone as ill be at work.