Confirm the iPhone is paid off and not under contract
Check that the device is eligible for carrier unlocking
Contact the carrier that originally sold the iPhone
Request a carrier unlock for the device
Provide the IMEI number when asked
Follow any carrier-specific verification steps
Wait for the carrier to approve the unlock
Insert a SIM card from a different carrier
Restart the iPhone if needed
If required, connect the iPhone to Wi-Fi or a computer to complete activation
If the carrier refuses, resolve any outstanding balance or account issue
If the carrier cannot unlock it, contact Apple for device status confirmation