How to unlock lost iphone

You can, but you have to turn off lost mode

1. In Find My iPhone on iCloud.com, click All Devices, then select the device that is in Lost Mode.
2. Click Lost Mode, click Stop Lost Mode, then click Stop Lost Mode again.
3. You can also turn off Lost Mode by entering your passcode on the device.

See this article for further reference: Use Lost Mode in Find My iPhone on iCloud.com – Apple Support (UK)