Lost Iphone

If they know your passcode, it wouldn't help, according to this support document. Use Lost Mode in Find My iPhone on iCloud.com - Apple Support To get the device out of Lost Mode, they would just need to enter your passcode. However, if you are worried about your content, but are not as worried about getting the iPhone back, you can remotely wipe the device to protect your data. Problem is, at that point, you can no longer track the device.

If you mean you got your carrier to block your device on the carrier's network based on the IMEI number, then whoever got possession of your iPhone would not be able to make calls over the carrier network, but they would still have access to the info on your iPhone if they knew the passcode.