I brought 2 iPhone 4S from Singapore and shipped to India for my brother and his friend. Apple Iphone always having trouble with network carrier if it is not factory Unlocked. So, I buy both Factory unlock iPhone 4S. However, got network problem with one iPhone 4S. My problem is one iphone 4 is perfectly working with Indian carrier and another cannot identify the Sim card. It shows sim card is Invalid. I remove the sim card and insert it again but cannot solve the problem. Even I tried different sim card but cannot solve the problem. I checked same Sim card is working perfect in one iphone and not in second one. I don’t understand why is not working in one phone. Anyone have any idea to solve this problem then please help.