I think he means that the hijackers call into T-Mobile customer support who then allow his number to be ported to their sim.
My T-Mobile number was simjacked last year, though afterwards once I reclaimed my number they let me set up a "secret word" that the person calling in has to give them and I haven't had any problems since.
I've asked that my number be unable to be changed to a different sim over the phone. So to change it I need to go into a physical store and present proper ID
My T-Mobile number was simjacked last year, though afterwards once I reclaimed my number they let me set up a "secret word" that the person calling in has to give them and I haven't had any problems since.