will not recognize sim card

Need help with this one... I just bought the phone from someone on Facebook. Well phone is unlocked and when I put my att sim card in it says inserted sim and restarts but still says no sim when it restarts. I called Sony and they told me to hook it to computer and download the software again so I did and it still has the same problem. I put my wife sim card it it it will read that there's a sim card inserted but will never give me cell service. I don't know what else to do. Please if anyone knows what I could try please let me know.. Thanks