I didn’t see in the article HOW they are doing that.
Your phone number is registered to your SIM card. Move the SIM card to another phone and the new phone rings when you dial the number.
The thief called the phone company and tricked them into registering the guy's phone number to a new SIM card in the thief's possession.
The thief called the phone company and tricked them into registering the guy’s phone number to a new SIM card in the thief’s possession.
Based on what you said, it is the phone company that was fooled here - and therefore responsible for damages.