I guess your phone is locked to particular Network.That's why it shows "Invalid Sim" To solve this problem, your need to get it unlock your phone. You can unlock your Nokia phone from Network lock using unlock code. Unlock code is the best and safest way to unlock phone. You can get the unlock code for your phone from any third party online vendors like http://www.wickedunlock.com/nokia-6681/rs16wp9/ here you need to specify your country, Network name to which it is locked, IMEI number of your phone. Then they deliver unlock code to you through mail with unlocking instructions. Using this you can unlock your phone from Network lock easily.
sahana, October 2013
