Hello panjgoori,
It seems that your sim is locked to network carrier. You need to unlock it. It is a network lock. If you forget your mobile's security code means only you can use default security code. So it seems that you have to break your network lock. Contact the network carrier and ask for unlock your mobile, or try to get unlock code for your mobile at any online service. To get unlock code you have to provide your mobile's IMEI number, country and network carrier it is locked to. If you wish to unlock your mobile means just go through the service Simpleunlocking.com and get unlock code for your mobile. Through this you can unlock your mobile at reliable price and within short time period. And follow the instructions at the same service. It'll be helpful to unlock your Nokia N900..