Why is my iPhone not accepting my SIM card?

SIM you currently have installed in this iPhone is from a network provider that is not supported under the activation policy currently assigned by the activation server.

This is not a hardware issue with the

iPhone.

Please install another SIM from a supported network provider or request that this iPhone be unlocked by your provider. Please contact Apple for more information.

How can i solve this problem please help me

[Re-Titled by Moderator]