Many mobile network companies lock mobile devices to their networks so that they cannot be accessed by any other cellular networks other than that of theirs. One cannot also use any free network sim cards in their phones if they have bought a phone that is locked with a particular mobile network. To be able to use your locked phone with other network carrier, you are required to unlock the phone with a code which is not an easy job. This is the reason why it is important to approach a professional to unlock your phone so that you can use the cheapest and the best sim card in your device.