Block Stolen iPhone

See the following: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If your phone had a passcode and you enabled Find my iPhone before it was stolen you just need to put it in Lost mode and it will be blocked. More effectively than using your carrier' IMEI block if they have one.

That would be done by the cellular carrier, if they do that. Understand, that if you block the phone, and/or have them turn off cellular, you will no longer be able to track the device.