nokia phone says insert sim card,but the sim card is in the phone?

Hi insert sim message will occur in two cases .If you haven't inserted the sim correctly in your phone then it will display insert sim message .Else if your Nokia phone is locked to some other network provider then it wont accept your sim until you unlock it.In this case if you insert other network sim then it will display insert sim ...To fix this you have to unlock your Nokia phone from network lock .You can easily unlock your phone using sites like Unlocking4Free.com they will generate network unlock code for some older mobile models at free of cost ....After unlocking your phone you can freely use it with any GSM sim .