How can I unlock the SIM card on my iPhone?

See these circumstances:

Unlock 1 = SIM not supported:-> How to unlock your iPhone for use with a different carrier - Apple Support

Unlock 2 = SIM PIN:->  Contact your carrier asking them for your SIM PUK code

Regards

Giulio

If your SIM is locked and needs a password (PIN) which you don't know, you need to contact your cellular network for assistance and the unlock code.